Source : Webster's Revised Unabridged Dictionary (1913)

Extremist \Ex*trem"ist\, n.
   A supporter of extreme doctrines or practice; one who holds
   extreme opinions.

Source : WordNet®

extremist
     adj : (used of opinions and actions) far beyond the norm;
           "extremist political views"; "radical opinions on
           education"; "an ultra conservative" [syn: {radical}, {ultra}]
     n : a person who holds extreme views