adjective

1. 1. of or from the root or roots; going to the foundation or source of something; fundamental; basic a radical principle
   2. extreme; thorough a radical change in one's life
2. 1. favoring fundamental or extreme change; specif., favoring basic change in the social or economic structure
   2. designating or of any of various modern political parties, esp. in Europe, ranging from moderate to conservative in program
3. Bot. of or coming from the root
4. Math. having to do with the root or roots of a number or quantity

Etymology: ME < LL radicalis < L radix (gen. radicis), root

noun

1. 1. a basic or root part of something
   2. a fundamental
2. 1. a person holding radical views, esp. one favoring fundamental social or economic change
   2. a member or adherent of a Radical party
3. Chem. a group of two or more atoms that acts as a single atom and goes through a reaction unchanged, or is replaced by a single atom: it is normally incapable of separate existence
4. Math.
   1. the indicated root of a quantity or quantities, shown by an expression written under the radical sign
   2. radical sign