The Collaborative International Dictionary of English v.0.48:

Aberrant \Ab*er"rant\, a. [L. aberrans, -rantis, p. pr. of
   aberrare. See Aberr.]
   1. Wandering; straying from the right way.
      [1913 Webster]

   2. (Biol.) Deviating from the ordinary or natural type;
      exceptional; abnormal.
      [1913 Webster]

            The more aberrant any form is, the greater must have
            been the number of connecting forms which, on my
            theory, have been exterminated.       --Darwin.
      [1913 Webster]

WordNet (r) 3.0 (2006):

aberrant
    adj 1: markedly different from an accepted norm; "aberrant
           behavior"; "deviant ideas" [syn: aberrant, deviant,
           deviate]
    n 1: one whose behavior departs substantially from the norm of a
         group