Do not confuse this word with radicle. (1) In chemistry, a group of elements or atoms usually passing intact from one compound to another, but usually incapable of prolonged existence in a free state (e.g., methyl, CH3); in chemical formulas, a radical is often distinguished by being enclosed in parentheses or brackets. (2) Thorough or extensive; relating or directed to the extirpation of the root or cause of a morbid process; e.g., a radical operation. (3) Denoting treatment by extreme, drastic, or innovative, as opposed to conservative, measures. (4) SYN: free radical. (5) A functional group in a molecule or molecular entity. [L. radix (radic-), root]