dichotomy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A division into two contrasting things or parts.
• noun Astronomy The phase of the moon, Mercury, or Venus when half of the disk is illuminated.
• noun Botany Branching characterized by successive forking into two approximately equal divisions.

from The Century Dictionary.

• noun A cutting in two; division into two parts or into twos; subdivision into halves or pairs; the state of being dichotomous.
• noun 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.
• noun In astronomy, that phase of the moon in which it appears bisected or shows only half its disk, as at the quadratures.
• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

• noun A cutting in two; a division.
• noun Division or distribution of genera into two species; division into two subordinate parts.
• noun (Astron.) That phase of the moon in which it appears bisected, or shows only half its disk, as at the quadratures.
• noun (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.
• noun The place where a stem or vein is forked.
• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A separation or division into two; a distinction that results in such a division.
• noun Such a division involving apparently incompatible or opposite principles; a duality.
• noun logic The division of a class into two disjoint subclasses that are together comprehensive, as the division of man into white and not white.
• noun biology, taxonomy The division of a genus into two species; a division into two subordinate parts.
• noun astronomy A phase of the moon when it appears half lit and half dark, as at the quadratures.
• noun 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.
• noun biology A fork (bifurcation) in a stem or vein.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun being twofold; a classification into two opposed parts or subclasses