dichotomy n, pl -mies [Gk dichotomia, fr. dichotomos] (1610)
1 : a division or the process of dividing into two esp. mutually exclusive or contradictory groups or entities
2 : the phase of the moon or an inferior planet in which half its disk appears illuminated
3 : a : BIFURCATION: esp : repeated bifurcation (as of a plant's stem)
b : a system of brancing in which the main axis forks repeatedly into two branches
c : branching of an ancestral line into two equal diverging branches
4 : something with seemingly contradictory qualities

Source: Merriam Webster Collegiate Dictionary, Tenth Edition.