<p><strong>Adjective</strong></p><ul><li>of or going to the root or origin; fundamental: There is a radical difference between the two interpretations of the sacred text, one justifying war and the other requiring nonviolent resistance.</li><li>extreme, especially as regards change from accepted or traditional forms: a radical change in the policy of the company.</li><li>favoring drastic political, economic, or social reforms: radical ideas; radical and anarchistic ideologues.</li><li>favoring, supporting, or representing extreme forms of religious fundamentalism: radical fundamentalists and their rejection of modern science.</li><li>forming a basis or foundation: What is the radical reason for his choice?</li><li>existing inherently in a thing or person: radical defects of character.</li><li>characterized by a marked departure from prevailing methods, practices, or ideas, particularly in the arts; experimental; unorthodox: The composer’s radical approach to sound, using timbre and rhythm rather than pitch, was declared “noise” by an influential critic.</li><li>slang: excellent; wonderful; cool; rad: You got here on an electric skateboard? That's radical!</li><li>pertaining to or forming a root.</li><li>denoting or pertaining to the radical sign.</li><li>irrational (def 5b).</li><li>of or pertaining to the root of a word.</li><li>of or arising from the root or the base of the stem.</li></ul><p><strong>Noun</strong></p><ul><li>a person who holds or follows strong convictions or extreme principles; extremist.</li><li>a person who advocates fundamental political, economic, and social reforms by direct and often uncompromising methods.</li><li>a quantity expressed as a root of another quantity.</li><li>the set of elements of a ring, some power of which is contained in a given ideal.</li><li>radical sign (def).</li><li>Chemistry: group (def 3); free radical (def).</li><li>Grammar: root 1 (def 12).</li><li>(in Chinese writing) one of 214 ideographic elements used in combination with phonetics to form thousands of different characters.</li></ul>