\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.

\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.