Webster's English Dictionary

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radical

1. rad.i.cal \'rad-i-k*l\ aj [ME, fr. LL radicalis, fr. L radic-, radix 
   root - more at (MROOT 1: of, relating to, or proceeding from a root  2: of 
   or relating to the origin : FUNDAMENTAL  3a: marked by a considerable 
   departure from the usual or traditional : (MEXTREME 3b: tending or disposed 
   to make extreme changes in existing views, habits,  conditions, or 
   institutions 3c: of, relating to, or constituting a political group 
   associated with vie ws, practices, and policies of extreme change - 
   rad.i.cal.ness n
2. radical n 1a: a root part  1b: basic principle : FOUNDATION  2a: ROOT  
   2b: a sound or letter belonging to a radical  3: one who is radical  4a: a 
   single replaceable atom or the reactive atomic form of an element  4b: a 
   group of atoms that is replaceable by a single atom, that is capable  of 
   remaining unchanged during a series of reactions, or that may show a 
   definite transitory existence in the course of a reaction 5a: RADICAL 
   EXPRESSION  5b: RADICAL SIGN