The Collaborative International Dictionary of English v.0.48:

Radical \Rad"i*cal\ (r[a^]d"[i^]*kal), a. [F., fr. L. radicalis
   having roots, fr. radix, -icis, a root. See Radix.]
   1. Of or pertaining to the root; proceeding directly from the
      root.
      [1913 Webster]

   2. Hence: Of or pertaining to the root or origin; reaching to
      the center, to the foundation, to the ultimate sources, to
      the principles, or the like; original; fundamental;
      thorough-going; unsparing; extreme; as, radical evils;
      radical reform; a radical party.
      [1913 Webster]

            The most determined exertions of that authority,
            against them, only showed their radical
            independence.                         --Burke.
      [1913 Webster]

   3. (Bot.)
      (a) Belonging to, or proceeding from, the root of a plant;
          as, radical tubers or hairs.
      (b) Proceeding from a rootlike stem, or one which does not
          rise above the ground; as, the radical leaves of the
          dandelion and the sidesaddle flower.
          [1913 Webster]

   4. (Philol.) Relating, or belonging, to the root, or ultimate
      source of derivation; as, a radical verbal form.
      [1913 Webster]

   5. (Math.) Of or pertaining to a radix or root; as, a radical
      quantity; a radical sign. See below.
      [1913 Webster]

   Radical axis of two circles. (Geom.) See under Axis.

   Radical pitch, the pitch or tone with which the utterance
      of a syllable begins. --Rush.

   Radical quantity (Alg.), a quantity to which the radical
      sign is prefixed; specifically, a quantity which is not a
      perfect power of the degree indicated by the radical sign;
      a surd.

   Radical sign (Math.), the sign [root] (originally the
      letter r, the initial of radix, root), placed before any
      quantity, denoting that its root is to be extracted; thus,
      [root]a, or [root](a + b). To indicate any other than the
      square root, a corresponding figure is placed over the
      sign; thus, [cuberoot]a, indicates the third or cube root
      of a.

   Radical stress (Elocution), force of utterance falling on
      the initial part of a syllable or sound.

   Radical vessels (Anat.), minute vessels which originate in
      the substance of the tissues.
      [1913 Webster]

   Syn: Primitive; original; natural; underived; fundamental;
        entire.

   Usage: Radical, Entire. These words are frequently
          employed as interchangeable in describing some marked
          alteration in the condition of things. There is,
          however, an obvious difference between them. A radical
          cure, reform, etc., is one which goes to the root of
          the thing in question; and it is entire, in the sense
          that, by affecting the root, it affects in an
          appropriate degree the entire body nourished by the
          root; but it may not be entire in the sense of making
          a change complete in its nature, as well as in its
          extent. Hence, we speak of a radical change; a radical
          improvement; radical differences of opinion; while an
          entire change, an entire improvement, an entire
          difference of opinion, might indicate more than was
          actually intended. A certain change may be both
          radical and entire, in every sense.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Radical \Rad"i*cal\ (r[a^]d"[i^]*kal), n.
   1. (Philol.)
      (a) A primitive word; a radix, root, or simple, underived,
          uncompounded word; an etymon.
      (b) A primitive letter; a letter that belongs to the
          radix.
          [1913 Webster]

                The words we at present make use of, and
                understand only by common agreement, assume a
                new air and life in the understanding, when you
                trace them to their radicals, where you find
                every word strongly stamped with nature; full of
                energy, meaning, character, painting, and
                poetry.                           --Cleland.
          [1913 Webster]

   2. (Politics) One who advocates radical changes in government
      or social institutions, especially such changes as are
      intended to level class inequalities; -- opposed to
      conservative.
      [1913 Webster]

            In politics they [the Independents] were, to use the
            phrase of their own time, "Root-and-Branch men," or,
            to use the kindred phrase of our own, Radicals.
                                                  --Macaulay.
      [1913 Webster]

   3. (Chem.)
      (a) A characteristic, essential, and fundamental
          constituent of any compound; hence, sometimes, an
          atom.
          [1913 Webster]

                As a general rule, the metallic atoms are basic
                radicals, while the nonmetallic atoms are acid
                radicals.                         --J. P. Cooke.
          [1913 Webster]
      (b) Specifically, a group of two or more atoms, not
          completely saturated, which are so linked that their
          union implies certain properties, and are conveniently
          regarded as playing the part of a single atom; a
          residue; -- called also a compound radical. Cf.
          Residue.
          [1913 Webster]

   4. (Alg.) A radical quantity. See under Radical, a.
      [1913 Webster]

            An indicated root of a perfect power of the degree
            indicated is not a radical but a rational quantity
            under a radical form.                 --Davies &
                                                  Peck (Math.
                                                  Dict.)
      [1913 Webster]

   5. (Anat.) A radical vessel. See under Radical, a.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Residue \Res"i*due\ (r?z"?-d?), n. [F. r['e]sidu, L. residuum,
   fr. residuus that is left behind, remaining, fr. residere to
   remain behind. See Reside, and cf. Residuum.]
   1. That which remains after a part is taken, separated,
      removed, or designated; remnant; remainder.
      [1913 Webster]

            The residue of them will I deliver to the sword.
                                                  --Jer. xv. 9.
      [1913 Webster]

            If church power had then prevailed over its victims,
            not a residue of English liberty would have been
            saved.                                --I. Taylor.
      [1913 Webster]

   2. (Law) That part of a testeator's estate wwhich is not
      disposed of in his will by particular and special legacies
      and devises, and which remains after payment of debts and
      legacies.
      [1913 Webster]

   3. (Chem.) That which remains of a molecule after the removal
      of a portion of its constituents; hence, an atom or group
      regarded as a portion of a molecule; a moiety or
      group; -- used as nearly equivalent to radical, but in
      a more general sense.
      [1913 Webster +PJC]

   Note: The term radical is sometimes restricted to groups
         containing carbon, the term residue and moiety being
         applied to the others.
         [1913 Webster]

   4. (Theory of Numbers) Any positive or negative number that
      differs from a given number by a multiple of a given
      modulus; thus, if 7 is the modulus, and 9 the given
      number, the numbers -5, 2, 16, 23, etc., are residues.
      [1913 Webster]

   Syn: Rest; remainder; remnant; balance; residuum; remains;
        leavings; relics.
        [1913 Webster]

WordNet (r) 3.0 (2006):

radical
    adj 1: (used of opinions and actions) far beyond the norm;
           "extremist political views"; "radical opinions on
           education"; "an ultra conservative" [syn: extremist,
           radical, ultra]
    2: markedly new or introducing radical change; "a revolutionary
       discovery"; "radical political views" [syn: revolutionary,
       radical]
    3: arising from or going to the root or source; "a radical flaw
       in the plan"
    4: of or relating to or constituting a linguistic root; "a
       radical verb form"
    5: especially of leaves; located at the base of a plant or stem;
       especially arising directly from the root or rootstock or a
       root-like stem; "basal placentation"; "radical leaves" [syn:
       radical, basal] [ant: cauline]
    n 1: (chemistry) two or more atoms bound together as a single
         unit and forming part of a molecule [syn: group,
         radical, chemical group]
    2: an atom or group of atoms with at least one unpaired
       electron; in the body it is usually an oxygen molecule that
       has lost an electron and will stabilize itself by stealing an
       electron from a nearby molecule; "in the body free radicals
       are high-energy particles that ricochet wildly and damage
       cells" [syn: free radical, radical]
    3: a person who has radical ideas or opinions
    4: (mathematics) a quantity expressed as the root of another
       quantity
    5: a character conveying the lexical meaning of a logogram
    6: (linguistics) the form of a word after all affixes are
       removed; "thematic vowels are part of the stem" [syn: root,
       root word, base, stem, theme, radical]

Moby Thesaurus II by Grady Ward, 1.0:

429 Moby Thesaurus words for "radical":
   Bolshevik, Bolshevist, Fabian, Gospel side, IC analysis, Jacobin,
   Kekule formula, Wobbly, ab ovo, aboriginal, absolute, accidence,
   acid, acidity, advanced, affix, affixation, agent, agitator,
   algorismic, algorithmic, aliquot, alkali, alkalinity,
   all-embracing, all-encompassing, all-inclusive, all-out,
   all-pervading, alloisomer, allomorph, anarchist, anarchistic,
   anarcho-syndicalist, anion, antacid, arrowhead, at the height,
   at the limit, atom, atomic cluster, basal, base, basement, basic,
   basilar, basis, bearing wall, bed, bedding, bedrock, beginning,
   benzene ring, biochemical, born, bottom, bound morpheme,
   branched chain, broad, broad-based, broad-minded, cantorial side,
   cardinal, cataclysmic, catastrophic, cation, central, chain,
   character, chemical, chemical element, chromoisomer, clean, clear,
   closed chain, commencement, communist, complete, compound,
   compound radical, comprehensive, conception, congenital,
   conjugation, constituent, constitutional, constitutive, consummate,
   copolymer, corrective, counterclockwise, crucial, cuneiform,
   cutting, cycle, decimal, declension, deep, deep-dyed, deep-seated,
   demotic character, derivation, determinative, difference of form,
   differential, digital, dimer, downright, drastic, dyad,
   dyed-in-the-wool, egregious, element, elemental, elementary,
   embryonic, emendatory, enclitic, entire, essential, even,
   exhaustive, exponential, extreme, extreme left-winger, extremist,
   extremistic, fanatic, fanatical, far out, figural, figurate,
   figurative, finite, floor, flooring, fond, footing, formative,
   foundation, foundational, fractional, free form, fundament,
   fundamental, furthest, generative, genesis, genetic, germinal,
   gradualistic, grammalogue, grass roots, greatest, ground, grounds,
   groundwork, gut, hardpan, head, heavy chemicals, heptad,
   heterocycle, hexad, hieratic symbol, hieroglyph, hieroglyphic,
   hieroglyphics, high polymer, hiragana, homocycle, homopolymer,
   hydracid, ideogram, ideograph, imaginary,
   immediate constituent analysis, immoderate, impair, impossible,
   in embryo, in ovo, inception, infinite, infix, infixation,
   inflection, inherent, inorganic chemical, insurgent,
   insurrectionary, insurrectionist, integral, intensive, intrinsic,
   ion, irrational, isomer, kana, katakana, larboard, lattice, left,
   left hand, left wing, left-hand, left-hand side, left-wing,
   left-wing extremist, left-winger, left-wingish, leftist,
   levorotatory, liberal, logarithmic, logogram, logograph,
   logometric, lunatic fringe, macromolecule, material, meliorist,
   melioristic, metamer, mild radical, mildly radical, militant,
   molecule, monad, monomer, morph, morpheme, morphemic analysis,
   morphemics, morphology, morphophonemics, most, natural, near,
   near side, negative, neutralizer, nigh, nihilist, nihilistic,
   nonacid, numeral, numerary, numerative, numeric, octad, odd,
   of the essence, ogham, omnibus, omnipresent, ordinal, organic,
   organic chemical, origin, original, origination, out-and-out,
   outright, oxyacid, pair, paradigm, parlor Bolshevik, parlor pink,
   pavement, pentad, perfect, pervasive, phonetic, phonetic symbol,
   pictogram, pictograph, pink, pinko, plain, plumb, polymer, port,
   port tack, portside, positive, possible, prefix, prefixation,
   pregnant, primal, primary, prime, primeval, primitive, primordial,
   principal, principle, pristine, proclitic, profound,
   progressionist, progressist, progressive, progressivist,
   protogenic, provenience, pseudoisomer, pure, rabid, radix,
   rational, reagent, rebel, reciprocal, red, reformational,
   reformative, reformatory, reformer, reformist, reformistic,
   regular, revisional, revisionist, revisory, revolutional,
   revolutionary, revolutionist, revulsionary, revulsive, ring,
   riprap, rise, rock bottom, root, rudiment, rudimentary, rune,
   sans-culotte, seat, secessionist, seminal, separatist, severe,
   sheer, shorthand, side chain, sill, simple radical, sinister,
   sinistral, sinistrocerebral, sinistrocular, sinistrogyrate,
   sinistrorse, solid ground, solid rock, source, space-lattice, stem,
   stereobate, stock, straight, straight chain, stylobate,
   submultiple, substantial, substantive, substratum, substruction,
   substructure, subversive, suffix, suffixation, sulfacid, surd,
   sweeping, syndicalist, taproot, terra firma, tetrad, theme,
   thorough, thoroughgoing, through-and-through, tolerant, too much,
   total, transcendental, transilient, triad, trimer, ubiquitous,
   ultra, ultra-ultra, ultraconservative, ultraist, ultraistic,
   unconditional, underbuilding, undercarriage, undergirding,
   underlying, underpinning, understruction, understructure,
   universal, unmitigated, unqualified, unreserved, unrestricted,
   utmost, utopian, utopist, utter, uttermost, veritable, verso,
   vital, way out, wedge, wholesale, wide, word letter,
   word-formation, wrong side, yippie, zealot