dichotomy

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Division into two usually contradictory parts or opinions: "the dichotomy of the one and the many” ( Louis Auchincloss).
2. n. Astronomy The phase of the moon, Mercury, or Venus when half of the disk is illuminated.
3. n. Botany Branching characterized by successive forking into two approximately equal divisions.

Century Dictionary and Cyclopedia

1. n. A cutting in two; division into two parts or into twos; subdivision into halves or pairs; the state of being dichotomous.
2. n. Specifically — In logic, the division of a whole into two parts; binary classification. Ramus revived, against the Aristotelians, the Platonic doctrine, which has had many adherents, that all classification should be by dichotomy. But the opinion has found little favor since Kant.
3. n. In astronomy, that phase of the moon in which it appears bisected or shows only half its disk, as at the quadratures.
4. n. In botany, a mode of branching by constant forking, as is shown in some stems, the venation of some leaves, etc. This mode of branching in plants is variously modified, as when only one of the branches at each fork becomes further developed, in which case the dichotomy is said to be sympodial. If these undeveloped branches lie always upon the same side of the axis, the sympodial dichotomy is helicoid; if alternately upon opposite sides, it is scorpioid.

Wiktionary

1. n. A separation or division into two; a distinction that results in such a division.
2. n. Such a division involving apparently incompatible or opposite principles; a duality.
3. n. logic The division of a class into two disjoint subclasses that are together comprehensive, as the division of man into white and not white.
4. n. biology, taxonomy The division of a genus into two species; a division into two subordinate parts.
5. n. astronomy A phase of the moon when it appears half lit and half dark, as at the quadratures.
6. n. biology Successive division and subdivision; successive bifurcation, as of a stem of a plant or a vein of the body into two parts as it proceeds from its origin.
7. n. biology A fork (bifurcation) in a stem or vein.

GNU Webster's 1913

1. n. A cutting in two; a division.
2. n. Division or distribution of genera into two species; division into two subordinate parts.
3. n. (Astron.) That phase of the moon in which it appears bisected, or shows only half its disk, as at the quadratures.
4. n. (Biol.) Successive division and subdivision, as of a stem of a plant or a vein of the body, into two parts as it proceeds from its origin; successive bifurcation.
5. n. The place where a stem or vein is forked.
6. n. (Logic) Division into two; especially, the division of a class into two subclasses opposed to each other by contradiction, as the division of the term man into white and not white.

WordNet 3.0

1. n. being twofold; a classification into two opposed parts or subclasses