dichotomy

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

• The dichotomy between maria and highlands dominates lunar mineralogy.

Such a division involving apparently incompatible or opposite principles; a duality.countableuncountable

• Slavery flourished, and absolute centralisation of power did not permit European-style feudalism until very late and only in some cases. “The Eurasian dichotomy of suzerain versus subject left little room in between.”

The division of a class into two disjoint subclasses that are together comprehensive, as the division of human into female and male.countableuncountable

• 2011, Tomasz A. Gorarzd, Jacek Krzaczkowski, The Complexity of Problems Connected with Two-Element Algebras, Paweł M. Idziak, Andrzej Wronski, Reports on Mathematical Logic: No. 46, page 92, One can ask if for any algebra the considered pro

The division of a genus into two species; a division into two subordinate parts.countableuncountable

A phase of the moon when it appears half lit and half dark, for example at the quadratures.countableuncountable

• 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.

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.countableuncountable

• 1969, J. F. Rigby, Permian Sphenopsids from Antarctica, Geological Survey Professional Paper 613-F, page F-9, In one forked leaf there is a distinct vein dichotomy, and the leaf boundary commences 1.5 mm above the dichotomy.