pair

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

• noun Two corresponding persons or items, similar in form or function and matched or associated.
• noun One object composed of two joined, similar parts that are dependent upon each other.
• noun Two persons who are married, engaged, or dating.
• noun Two persons who have something in common and are considered together.
• noun Two mated animals.
• noun Two animals joined together in work.
• noun Games Two playing cards of the same denomination.
• noun Two members of a deliberative body with opposing opinions on a given issue who agree to abstain from voting on the issue, thereby offsetting each other.
• noun Chemistry An electron pair.
• intransitive verb To arrange in sets of two; couple.
• intransitive verb To combine or join (one person or thing) with another to form a pair.
• intransitive verb To form pairs or a pair.
• intransitive verb To join with another in love or mating.

from The Century Dictionary.

• To impair.
• To become impaired; deteriorate.
• To form a pair or pairs; specifically, to be joined in pairs as birds are in the breeding season; mate; couple.
• To suit; fit; match.
• To abstain from voting by arrangement with a member of the opposite party to do the same: said of members of deliberative assemblies. See pairing.
• To join in couples; specifically, to cause to mate: as, to pair a canary with a siskin.
• To unite or assort in twos as well suited to each other.
• In cribbage, to match (the card last played by the adversary).
• noun In roulette, an even number.
• noun In mech., two parts or pieces, each of which acts against the other to hold it in position or to restrain its motion, as a bearing and journal, or a screw and nut.
• noun Two things of a kind, similar in form, identical in purpose, and matched or used together: as, a pair of gloves; a pair of shoes.
• noun A single thing composed essentially of two pieces or parts which are used only in combination and named only in the plural: as, a pair of scissors, trousers, or spectacles.
• noun A couple; a brace; a span: as, a pair of pistols; a pair of horses.
• noun Specifically
• noun A married couple; in general, two mated animals of any kind.
• noun A set of like or equal things: restricted to a few (mostly obsolete) phrases: as, a pair (or pack) of cards; a pair (or flight) of stairs; a pair of organs (that is, a set of organ-pipes, hence an organ); a pair of gallows (that is, a gibbet); a pair of beads (see bead).
• noun In archery, a set of three arrows.
• noun In mining, a set or gang of men working together at the same hours.
• noun In deliberative bodies, two members belonging to opposing parties who for their own convenience (as to permit one or both of them to be absent) arrange with each other to refrain from voting for a specified time or on a specified question, thus nullifying a vote on each side; also, the arrangement thus effected. See pairing.
• noun In poker, two of the same denomination, without regard to suit or color: as, a pair of aces or deuces.
• noun See the adjectives.
• noun =Syn. 1-3. Pair, Couple, Yoke, Brace, Dyad, Duad. Pair and couple properly express two individuals or unities naturally or habitually going together or making a set: as, a pair of horses, gloves, oars; a wedded pair; a loving couple; but pair also means two things alike and put together, and couple has by colloquial use come to be often applied to two, however accidentally brought together: as, give him a couple of apples. Yoke, on the other hand, applies only to two animals customarily yoked together: as, a yoke of oxen. Brace is rather a hunters' term, with limited and peculiar application: as, a brace of partridges, pistols, slugs. Dyad is used in philosophical and mathematical language only. Duad is a special mathematical word signifying an unordered pair.

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

• transitive verb To unite in couples; to form a pair of; to bring together, as things which belong together, or which complement, or are adapted to one another.
• transitive verb Parliamentary Cant To engage (one's self) with another of opposite opinions not to vote on a particular question or class of questions.
• transitive verb (Zoöl.) See under Fin.
• intransitive verb To be joined in pairs; to couple; to mate, as for breeding.
• intransitive verb To suit; to fit, as a counterpart.
• intransitive verb Same as To pair off. See phrase below.
• intransitive verb Parliamentary Cant to agree with one of the opposite party or opinion to abstain from voting on specified questions or issues. See Pair, n., 6.
• transitive verb obsolete To impair.
• noun A number of things resembling one another, or belonging together; a set. “A pair of beads.” Chaucer. Beau. & Fl. “Four pair of stairs.” Macaulay. [Now mostly or quite disused.]
• noun Two things of a kind, similar in form, suited to each other, and intended to be used together
• noun Two of a sort; a span; a yoke; a couple; a brace
• noun A married couple; a man and wife.
• noun A single thing, composed of two pieces fitted to each other and used together
• noun Parliamentary Cant Two members of opposite parties or opinion, as in a parliamentary body, who mutually agree not to vote on a given question (in order, for example, to allow the members to be absent during the vote without affecting the outcome of the vote), or on issues of a party nature during a specified time.