RAD'ICAL, a. [L. radicalis, from radix, root. See Race and Ray.]

1. Pertaining to the root or origin; original; fundamental; as, a radical truth or error; a radical evil; a radical difference of opinions or systems.
2. Implanted by nature; native; constitutional; as, the radical moisture of a body. – Bacon.
3. Primitive; original; underived; uncompounded; as, a radical word.
4. Serving to origination.
5. In botany, proceeding immediately from the root; as, a radical leaf or peduncle. – Martyn.

1. In philology, a primitive word; a radix, root, or simple underived uncompounded word.
2. A primitive letter; a letter that belongs to the radix.
3. In modern politics, a person who advocates a radical reform, or extreme measures in reformation.
4. In chimistry, an element, or a simple constituent part of a substance, which is incapable of decomposition. – Parke. That which constitutes the distinguishing part of an acid or a base, by its union with oxygen, or other acidifying and basifying principles. – Ure. Compound radical, is a base composed of two or more substances. Thus a vegetable acid having a radical composed of hydrogen and carbon, is said to be an acid with a compound radical. Radical quantities, in algebra, quantities whose roots may be accurately expressed in numbers. The term is sometimes extended to all quantities under the radical sign. Radical sign, the sign √ placed before any quantity, denoting that its root is to be extracted; thus, √a or √(a+b). – Encyc. Bailey.

1. Of or pertaining to the root; proceeding directly from the root.
2. A primitive word; a radix, root, or simple, underived, uncompounded word; an etymon.
   
   
   (b)
3. 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.

4. 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 the phrase of their own time, "Root-and-Branch men," or, to use the kindred phrase of our own, Radicals. Macaulay.

5. Belonging to, or proceeding from, the root of a plant; as, radical tubers or hairs.
   
   
   (b)
6. 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)

7. Relating, or belonging, to the root, or ultimate source of derivation; as, a radical verbal form.
8. 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.)

9. 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 ***radic] (originally the letter r, the initial of radix, root), placed before any quantity, denoting that its root is to be extracted; thus, ***radic]a, or ***radic](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. -- 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.

10. A radical vessel. See under Radical, a.