dichotomy

noun6

1. •
   A separation or division into two; a distinction that results in such a division.

   • The dichotomy between maria and highlands dominates lunar mineralogy. — 2003
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 human into female and male.
4. •
   The division of a genus into two species; a division into two subordinate parts.
5. •
   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.

   • In one forked leaf there is a distinct vein dichotomy, and the leaf boundary commences 1.5 mm above the dichotomy. — 1969
6. •
   A phase of the moon when it appears half lit and half dark, for example at the quadratures.

   • The Ides of Januarius indeed, the preceding month, must have fallen on March 1 at midnight, two days before the first dichotomy of the mean new moon of that month, March 3 at midnight. — 1854