Source : Webster's Revised Unabridged Dictionary (1913)

Atypic \A*typ"ic\, Atypical \A*typ"ic*al\, a. [Pref. a- not +
   typic, typical.]
   That has no type; devoid of typical character; irregular;
   unlike the type.

Source : WordNet®

atypical
     adj 1: deviating from or not conforming to type; "atypical
            behavior..not the accepted type of response that we
            expect from children"- G.E.Gardner [syn: {untypical}]
            [ant: {typical}]
     2: deviating from what is usual or common or to be expected;
        often somewhat odd or strange; "these days large families
        are atypical"; "highly irregular behavior" [syn: {irregular}]
     3: not conforming to type; "showed atypical agility"