Source : Webster's Revised Unabridged Dictionary (1913)

Radical \Rad"i*cal\, 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.

   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.

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

   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.

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

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

   {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.

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

   Usage: {Radical}, {Entire}. These words are frequently
          employed as interchangeable in describing some marked
          alternation 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 a
          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.

Radical \Rad"i*cal\, 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.

                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.

   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.

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

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

                As a general rule, the metallic atoms are basic
                radicals, while the nonmetallic atoms are acid
                radicals.                         --J. P. Cooke.
      (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}.

   4. (Alg.) A radical quantity. See under {Radical}, a.

            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.)

   5. (Anat.) A radical vessel. See under {Radical}, a.

Source : WordNet®

radical
     adj 1: (used of opinions and actions) far beyond the norm;
            "extremist political views"; "radical opinions on
            education"; "an ultra conservative" [syn: {extremist},
             {ultra}]
     2: markedly new or introducing radical change; "a revolutionary
        discovery"; "radical political views" [syn: {revolutionary}]
     3: arising from or going to the root; "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: {basal}] [ant: {cauline}]

radical
     n 1: (chemistry) two or more atoms bound together as a single
          unit and forming part of a molecule [syn: {group}, {chemical
          group}]
     2: an atom or group of atoms with at least one unpaired
        electron; in the body it is usually an oxygen molecule
        than 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}]
     3: a person who has radical ideas or opinions
     4: a character conveying the lexical meaning of a logogram
     5: a sign placed in front of an expression to denote that a
        root is to be extracted [syn: {radical sign}]
     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}]