WordWeb Online

1. Not representative of a group, class, or type
   "a group that is atypical of the target audience"; "a class of atypical mosses"; "atypical behaviour is not the accepted type of response that we expect from children"
   - untypical
    
2. Deviating from normal expectations; somewhat odd, strange, or abnormal
   "these days large families are atypical"; "atypical clinical findings"; "atypical pneumonia"
   - irregular

See also: abnormal, atypicality, typicality, uncharacteristic, unnatural, unrepresentative

Antonym: typical