dichotomous

from WordNet (r) 3.0 (2006)

dichotomous
    adj 1: divided or dividing into two sharply distinguished parts
           or classifications

from The Collaborative International Dictionary of English v.0.48

Dichotomous \Di*chot"o*mous\, a. [L. dichotomos, Gr. ?; ? in
   two, asunder + diate`mnein to cut.]
   Regularly dividing by pairs from bottom to top; as, a
   dichotomous stem. -- {Di*chot"o*mous*ly}, adv.
   [1913 Webster]