Dichotomies

noun

1. A separation or division into two; a distinction that results in such a division.
2. Such a division involving apparently incompatible or opposite principles; a duality.
3. The division of a class into two disjoint subclasses that are together comprehensive, as the division of man into white and not white.
4. The division of a genus into two species; a division into two subordinate parts.
5. A phase of the moon when it appears half lit and half dark, as at the quadratures.