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.
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   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.
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            The most determined exertions of that authority,
            against them, only showed their radical
            independence.                         --Burke.
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   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.
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   4. (Philol.) Relating, or belonging, to the root, or ultimate
      source of derivation; as, a radical verbal form.
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   5. (Math.) Of or pertaining to a radix or root; as, a radical
      quantity; a radical sign. See below.
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   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.
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   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.
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                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.
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   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.
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            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.
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   3. (Chem.)
      (a) A characteristic, essential, and fundamental
          constituent of any compound; hence, sometimes, an
          atom.
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                As a general rule, the metallic atoms are basic
                radicals, while the nonmetallic atoms are acid
                radicals.                         --J. P. Cooke.
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      (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.
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   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.)
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   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.
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            The residue of them will I deliver to the sword.
                                                  --Jer. xv. 9.
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            If church power had then prevailed over its victims,
            not a residue of English liberty would have been
            saved.                                --I. Taylor.
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   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.
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   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.
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   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.
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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]