noun

• A separation or division into two; a distinction that results in such a division.
• Such a division involving apparently incompatible or opposite principles; a duality.
• (logic) The division of a class into two disjoint subclasses that are together comprehensive, as the division of human into female and male.
• (biology, taxonomy) The division of a genus into two species; a division into two subordinate parts.
• (astronomy) A phase of the moon when it appears half lit and half dark, for example at the quadratures.
• (botany) 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.