The Collaborative International Dictionary of English v.0.48:

Half tone \Half tone\, or Half-tone \Half"-tone`\, n.
   1. (Fine Arts) An intermediate or middle tone in a painting,
      engraving, photograph, etc.; a middle tint, neither very
      dark nor very light.
      [Webster 1913 Suppl.]

   2. (Music) A half step.
      [Webster 1913 Suppl.]

   3. A print obtained by the half-tone photo-engraving process.
      [WordNet sense 1]
      [PJC]

   4. the etched plate used to reproduce a half-tone
      illustration. [WordNet sense 4]

   Syn: halftone engraving, photoengraving.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Half-tone \Half"-tone`\ (h[aum]f"t[=o]n`), a.
   Having, consisting of, or pertaining to, half tones; specif.
   (Photo-engraving), pertaining to or designating plates,
   processes, or the pictures made by them, in which gradation
   of tone in the photograph is reproduced by a graduated system
   of dotted and checkered spots, usually nearly invisible to
   the unaided eye, produced by the interposition between the
   camera and the object of a screen. The name alludes to the
   fact that this process was the first that was practically
   successful in reproducing the half tones of the photograph.
   [Webster 1913 Suppl.]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "halftone":
   Maxwell triangle, Munsell scale, chromatic circle,
   chromatic spectrum, chromaticity diagram, color circle,
   color cycle, color index, color mixture curve, color solid,
   color spectrum, color system, color triangle, complementary color,
   consecutive intervals, degree, demitint, diapason, diatessaron,
   diatonic interval, diatonic semitone, enharmonic diesis,
   enharmonic interval, fifth, fourth, full color, fundamental colors,
   half step, half tint, hue cycle, interval, less semitone,
   melodic interval, metamer, monochrome, note, octave,
   parallel octaves, primary, primary color, pure color, second,
   secondary, secondary color, semitone, seventh, sixth,
   solar spectrum, spectral color, spectrum, spectrum color, step,
   tertiary, tertiary color, third, tone, unison interval,
   whole step