The Collaborative International Dictionary of English v.0.48:

Unisexual \U`ni*sex"u*al\, a. [Uni- + sexual: cf. F. unisexuel.]
   (Biol.)
   Having one sex only, as plants which have the male and female
   flowers on separate individuals, or animals in which the
   sexes are in separate individuals; di[oe]cious; --
   distinguished from bisexual, or hermaphrodite. See
   Di[oe]cious.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unisexual
    adj 1: relating to only one sex or having only one type of
           sexual organ; not hermaphroditic