dichotomy

Short definition from Wiktionary

Noun

1. 1
   A separation or division into two; a distinction that results in such a division.
2. 2
   Such a division involving apparently incompatible or opposite principles; a duality.
3. 3
   The division of a class into two disjoint subclasses that are together comprehensive, as the division of human into female and male.
4. 4
   The division of a genus into two species; a division into two subordinate parts.
5. 5
   A phase of the moon when it appears half lit and half dark, for example at the quadratures.
6. 6
   Division and subdivision; bifurcation, such as that of a stem of a plant or a vein of the body into two parts as it proceeds from its origin; often successive.