What does "dichotomy" mean?

Additional senses

1. 2.(Astron.) That phase of the moon in which it appears bisected, or shows only half its disk, as at the quadratures.
2. 3.(Biol.) Successive division and subdivision, as of a stem of a plant or a vein of the body, into two parts as it proceeds from its origin; successive bifurcation.
3. 4.The place where a stem or vein is forked.
4. 5.(Logic) Division into two; especially, the division of a class into two subclasses opposed to each other by contradiction, as the division of the term man into white and not white.