The Collaborative International Dictionary of English v.0.48:

Tone \Tone\ (t[=o]n), n. [F. ton, L. tonus a sound, tone, fr.
   Gr. to`nos a stretching, straining, raising of the voice,
   pitch, accent, measure or meter, in pl., modes or keys
   differing in pitch; akin to tei`nein to stretch or strain.
   See Thin, and cf. Monotonous, Thunder, Ton fashion,
   Tune.]
   1. Sound, or the character of a sound, or a sound considered
      as of this or that character; as, a low, high, loud,
      grave, acute, sweet, or harsh tone.
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            [Harmony divine] smooths her charming tones.
                                                  --Milton.
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            Tones that with seraph hymns might blend. --Keble.
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   2. (Rhet.) Accent, or inflection or modulation of the voice,
      as adapted to express emotion or passion.
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            Eager his tone, and ardent were his eyes. --Dryden.
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   3. A whining style of speaking; a kind of mournful or
      artificial strain of voice; an affected speaking with a
      measured rhythm ahd a regular rise and fall of the voice;
      as, children often read with a tone.
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   4. (Mus.)
      (a) A sound considered as to pitch; as, the seven tones of
          the octave; she has good high tones.
      (b) The larger kind of interval between contiguous sounds
          in the diatonic scale, the smaller being called a
          semitone as, a whole tone too flat; raise it a tone.
      (c) The peculiar quality of sound in any voice or
          instrument; as, a rich tone, a reedy tone.
      (d) A mode or tune or plain chant; as, the Gregorian
          tones.
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   Note: The use of the word tone, both for a sound and for the
         interval between two sounds or tones, is confusing, but
         is common -- almost universal.
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   Note: Nearly every musical sound is composite, consisting of
         several simultaneous tones having different rates of
         vibration according to fixed laws, which depend upon
         the nature of the vibrating body and the mode of
         excitation. The components (of a composite sound) are
         called partial tones; that one having the lowest rate
         of vibration is the fundamental tone, and the other
         partial tones are called harmonics, or overtones. The
         vibration ratios of the partial tones composing any
         sound are expressed by all, or by a part, of the
         numbers in the series 1, 2, 3, 4, 5, etc.; and the
         quality of any sound (the tone color) is due in part to
         the presence or absence of overtones as represented in
         this series, and in part to the greater or less
         intensity of those present as compared with the
         fundamental tone and with one another. Resultant tones,
         combination tones, summation tones, difference tones,
         Tartini's tones (terms only in part synonymous) are
         produced by the simultaneous sounding of two or more
         primary (simple or composite) tones.
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   5. (Med.) That state of a body, or of any of its organs or
      parts, in which the animal functions are healthy and
      performed with due vigor.
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   Note: In this sense, the word is metaphorically applied to
         character or faculties, intellectual and moral; as, his
         mind has lost its tone.
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   6. (Physiol.) Tonicity; as, arterial tone.
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   7. State of mind; temper; mood.
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            The strange situation I am in and the melancholy
            state of public affairs, . . . drag the mind down .
            . . from a philosophical tone or temper, to the
            drudgery of private and public business.
                                                  --Bolingbroke.
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            Their tone was dissatisfied, almost menacing. --W.
                                                  C. Bryant.
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   8. Tenor; character; spirit; drift; as, the tone of his
      remarks was commendatory.
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   9. General or prevailing character or style, as of morals,
      manners, or sentiment, in reference to a scale of high and
      low; as, a low tone of morals; a tone of elevated
      sentiment; a courtly tone of manners.
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   10. The general effect of a picture produced by the
       combination of light and shade, together with color in
       the case of a painting; -- commonly used in a favorable
       sense; as, this picture has tone.
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   11. (Physiol.) Quality, with respect to attendant feeling;
       the more or less variable complex of emotion accompanying
       and characterizing a sensation or a conceptual state; as,
       feeling tone; color tone.
       [Webster 1913 Suppl.]

   12. Color quality proper; -- called also hue. Also, a
       gradation of color, either a hue, or a tint or shade.

             She was dressed in a soft cloth of a gray tone.
                                                  --Sir G.
                                                  Parker.
       [Webster 1913 Suppl.]

   13. (Plant Physiol.) The condition of normal balance of a
       healthy plant in its relations to light, heat, and
       moisture.
       [Webster 1913 Suppl.]

   Tone color. (Mus.) see the Note under def. 4, above.

   Tone syllable, an accented syllable. --M. Stuart.
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The Collaborative International Dictionary of English v.0.48:

Tone \Tone\, v. t. [imp. & p. p. Toned; p. pr. & vb. n.
   Toning.]
   1. To utter with an affected tone.
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   2. To give tone, or a particular tone, to; to tune. See
      Tune, v. t.
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   3. (Photog.) To bring, as a print, to a certain required
      shade of color, as by chemical treatment.
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   To tone down.
      (a) To cause to give lower tone or sound; to give a lower
          tone to.
      (b) (Paint.) To modify, as color, by making it less
          brilliant or less crude; to modify, as a composition
          of color, by making it more harmonius.
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                Its thousand hues toned down harmoniusly. --C.
                                                  Kingsley.
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      (c) Fig.: To moderate or relax; to diminish or weaken the
          striking characteristics of; to soften.
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                The best method for the purpose in hand was to
                employ some one of a character and position
                suited to get possession of their confidence,
                and then use it to tone down their religious
                strictures.                       --Palfrey.
          [1913 Webster]

   To tone up, to cause to give a higher tone or sound; to
      give a higher tone to; to make more intense; to heighten;
      to strengthen.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tone
    n 1: the quality of a person's voice; "he began in a
         conversational tone"; "he spoke in a nervous tone of voice"
         [syn: tone, tone of voice]
    2: (linguistics) a pitch or change in pitch of the voice that
       serves to distinguish words in tonal languages; "the Beijing
       dialect uses four tones"
    3: (music) the distinctive property of a complex sound (a voice
       or noise or musical sound); "the timbre of her soprano was
       rich and lovely"; "the muffled tones of the broken bell
       summoned them to meet" [syn: timbre, timber, quality,
       tone]
    4: the general atmosphere of a place or situation and the effect
       that it has on people; "the feel of the city excited him"; "a
       clergyman improved the tone of the meeting"; "it had the
       smell of treason" [syn: spirit, tone, feel, feeling,
       flavor, flavour, look, smell]
    5: a quality of a given color that differs slightly from another
       color; "after several trials he mixed the shade of pink that
       she wanted" [syn: shade, tint, tincture, tone]
    6: a notation representing the pitch and duration of a musical
       sound; "the singer held the note too long" [syn: note,
       musical note, tone]
    7: a steady sound without overtones; "they tested his hearing
       with pure tones of different frequencies" [syn: tone, pure
       tone]
    8: the elastic tension of living muscles, arteries, etc. that
       facilitate response to stimuli; "the doctor tested my
       tonicity" [syn: tonicity, tonus, tone] [ant:
       amyotonia, atonia, atonicity, atony]
    9: a musical interval of two semitones [syn: tone, whole
       tone, step, whole step]
    10: the quality of something (an act or a piece of writing) that
        reveals the attitudes and presuppositions of the author;
        "the general tone of articles appearing in the newspapers is
        that the government should withdraw"; "from the tone of her
        behavior I gathered that I had outstayed my welcome"
    v 1: utter monotonously and repetitively and rhythmically; "The
         students chanted the same slogan over and over again" [syn:
         tone, chant, intone]
    2: vary the pitch of one's speech [syn: tone, inflect,
       modulate]
    3: change the color or tone of; "tone a negative"
    4: change to a color image; "tone a photographic image"
    5: give a healthy elasticity to; "Let's tone our muscles" [syn:
       tone, tone up, strengthen]

Moby Thesaurus II by Grady Ward, 1.0:

488 Moby Thesaurus words for "tone":
   French pitch, Munsell chroma, Zeitgeist, accent, accidental,
   achromatism, action, actions, activity, acts, adaptability,
   address, affect, affectation, air, algorithm, anagnorisis, angle,
   apply paint, approach, architectonics, architecture, argument,
   arrangement, aspect, atmosphere, attack, attitude, audio frequency,
   aura, background, balance, bearing, bedaub, bedizen, beef,
   beefiness, begild, behavior, behavior pattern, behavioral norm,
   behavioral science, bend, besmear, bias, blend, body-build, bounce,
   bounciness, brand, brawn, brawniness, breve, brighten up,
   brightness, brush on paint, brushwork, buoyance, buoyancy,
   calcimine, carriage, cast, catastrophe, character, characteristic,
   characteristics, characterization, chroma, chromatic color,
   chromaticity, chromatism, chromism, classical pitch, climate, coat,
   color, color balance, color harmony, color quality, color scheme,
   coloration, colorimetric quality, coloring, complexion,
   complication, comportment, composition, conduct,
   consecutive intervals, constituents, constitution, continuity,
   contrivance, cool color, course, cover, crasis, crotchet, cue,
   culture pattern, current, custom, dab, dampen, daub,
   decorator color, deep-dye, degree, demeanor, demisemiquaver,
   denouement, deportment, depth, design, development, device, dharma,
   diapason, diatessaron, diathesis, diatonic interval,
   diatonic semitone, dip, direction, dispose, disposition, distemper,
   doing, doings, dominant, dominant note, double whole note,
   double-dye, draftsmanship, drift, dulcetness, dull, dye,
   eighth note, elasticity, emblazon, emphasis, enamel, engild,
   enharmonic, enharmonic diesis, enharmonic interval,
   enharmonic note, episode, ethos, extensibility, fable, face,
   falling action, fashion, fast-dye, feel, feeling, fiber, fifth,
   flat, flexibility, folkway, force, form, fourth, frame,
   frame of mind, frequency, fresco, freshen up, fullness,
   fundamental, fundamental tone, genius, gestures, gild, gimmick,
   give, glacial movement, glaze, gloss, goings-on, grain, grouping,
   guise, habit, half note, half step, halftone, harmonic, heart,
   heftiness, height, hemidemisemiquaver, high pitch, hue, humor,
   humors, huskiness, ilk, illuminate, imbue, incident, incline,
   induce, inflection, influence, ingrain, interval, intonation,
   intonation pattern, involuntary muscle, japan, key, kind, lacquer,
   lay on color, lead, less semitone, lightness, limber up, line,
   line of action, lines, liveliness, local color, low pitch,
   main current, mainstream, maintien, makeup, manner,
   manner of speaking, manner of working, manners, means,
   mellifluence, mellifluousness, melodic interval, melodiousness,
   melody, method, methodology, methods, mien, milieu, mind, minim,
   mode, mode of expression, mode of operation, mode of procedure,
   moderate, modify, modulate, modulation, modus operandi,
   modus vivendi, mold, monotone, monotony, mood, morale, motif,
   motion, motions, move, movement, movements, moves, muscle,
   muscularity, musculature, musical note, musical quality,
   musical sound, musicality, mute, mythos, natural, natural color,
   nature, neutral color, new philharmonic pitch, note,
   observable behavior, octave, order, overtone, paint, painterliness,
   pallor, parallel octaves, parget, partial, partial tone,
   patent note, pattern, peripeteia, perspective, persuade,
   philharmonic pitch, philosophical pitch, phrasing, physique,
   pigment, pitch, pitch accent, plan, plot, poise, port, pose,
   posture, practice, praxis, predispose, presence, prime, procedure,
   proceeding, process, prompt, property, purity, quality,
   quarter note, quaver, quieten, rebound, recognition, reduce,
   register, reinvigorate, report, resilience, resiliency, resonance,
   responding note, responsiveness, revitalize, richness,
   rising action, routine, run, saturation, scheme, second,
   secondary plot, semibreve, semiquaver, semitone, sense, set,
   seventh, shade, shading, shadow, shape, shaped note, sharp,
   shellac, sinew, sinewiness, sinews, sixteenth note, sixth,
   sixty-fourth note, skin color, slant, slop on paint, smear, snap,
   social science, soft-pedal, soften, soften up, somatotype,
   sonority, sonorousness, sort, sound, speech tune, spiccato, spirit,
   spirits, spring, springiness, staccato, stain, stamp,
   standard pitch, state of mind, step, stipple, story, streak,
   stream, stress, stretch, stretchability, stretchiness, stripe,
   structure, style, subdue, subject, subplot, suchness,
   suprasegmental, sustained note, sway, sweetness, swing, switch,
   system, tack, tactics, technique, temper, temperament, tendency,
   tenor, tercet, the drill, the general tendency, the how,
   the main course, the way of, thematic development, theme, thew,
   thewiness, thews, third, thirty-second note, timbre, time spirit,
   tinct, tincture, tinge, tint, tonality, tone color, tone down,
   tone of voice, tone quality, tone up, tonelessness, tonicity,
   tonus, topic, treatment, trend, triplet, tune, tune up,
   tunefulness, turn, twist, type, undercoat, undercolor, undertone,
   unison interval, value, values, varnish, vein, voce, voice,
   voice qualifier, voice quality, voluntary muscle, warm color, wash,
   way, way of life, way of saying, ways, wear down, weigh with,
   whitewash, whole note, whole step, wise, work



The Free On-line Dictionary of Computing (30 December 2018):

brightness
luminance
tone
value

    (Or "tone", "luminance", "value", "luminosity",
   "lightness") The coordinate in the HSB colour model that
   determines the total amount of light in the colour.  Zero
   brightness is black and 100% is white, intermediate values are
   "light" or "dark" colours.

   The other coordinates are hue and saturation.

   (1999-07-05)