The Collaborative International Dictionary of English v.0.48:

Pair \Pair\ (p[^a]r), n. [F. paire, LL. paria, L. paria, pl. of
   par pair, fr. par, adj., equal. Cf. Apparel, Par
   equality, Peer an equal.]
   [1913 Webster]
   1. A number of things resembling one another, or belonging
      together; a set; as, a pair or flight of stairs. "A pair
      of beads." --Chaucer. --Beau. & Fl. "Four pair of stairs."
      --Macaulay.

   Note: [Now mostly or quite disused.]
         [1913 Webster]

               Two crowns in my pocket, two pair of cards.
                                                  --Beau. & Fl.
         [1913 Webster]

   2. Two things of a kind, similar in form, suited to each
      other, and intended to be used together; as, a pair of
      gloves or stockings; a pair of shoes.
      [1913 Webster]

   3. Two of a sort; a span; a yoke; a couple; a brace; as, a
      pair of horses; a pair of oxen.
      [1913 Webster]

   4. A married couple; a man and wife. "A happy pair."
      --Dryden. "The hapless pair." --Milton.
      [1913 Webster]

   5. A single thing, composed of two pieces fitted to each
      other and used together; as, a pair of scissors; a pair of
      pants; a pair of tongs; a pair of bellows.
      [1913 Webster]

   6. 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; as, there were two pairs on the
      final vote. [Parliamentary Cant]

   Note: A member who is thus paired with one who would have
         voted oppositely is said to be paired for or paired
         against a measure, depending on the member's position.
         [1913 Webster +PJC]

   7. (Kinematics) In a mechanism, two elements, or bodies,
      which are so applied to each other as to mutually
      constrain relative motion.
      [1913 Webster]

   Note: Pairs are named in accordance with the kind of motion
         they permit; thus, a journal and its bearing form a
         turning pair, a cylinder and its piston a sliding
         pair, a screw and its nut a twisting pair, etc. Any
         pair in which the constraining contact is along lines
         or at points only (as a cam and roller acting
         together), is designated a higher pair; any pair
         having constraining surfaces which fit each other (as a
         cylindrical pin and eye, a screw and its nut, etc.), is
         called a lower pair.
         [1913 Webster]

   Pair royal (pl. Pairs Royal) three things of a sort; --
      used especially of playing cards in some games, as
      cribbage; as three kings, three "eight spots" etc. Four of
      a kind are called a double pair royal. "Something in his
      face gave me as much pleasure as a pair royal of naturals
      in my own hand." --Goldsmith. "That great pair royal of
      adamantine sisters [the Fates]." --Quarles. [Written
      corruptly parial and prial.]
      [1913 Webster]

   Syn: Pair, Flight, Set.

   Usage: Originally, pair was not confined to two things, but
          was applied to any number of equal things (pares),
          that go together. Ben Jonson speaks of a pair (set) of
          chessmen; also, he and Lord Bacon speak of a pair
          (pack) of cards. A "pair of stairs" is still in
          popular use, as well as the later expression, "flight
          of stairs."
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pair \Pair\, v. t. [See Impair.]
   To impair. [Obs.] --Spenser.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pair \Pair\, v. i. [imp. & p. p. Paired; p. pr. & vb. n.
   Pairing.]
   1. To be joined in pairs; to couple; to mate, as for
      breeding.
      [1913 Webster]

   2. To suit; to fit, as a counterpart.
      [1913 Webster]

            My heart was made to fit and pair with thine.
                                                  --Rowe.
      [1913 Webster]

   3. Same as To pair off. See phrase below.
      [1913 Webster]

   To pair off, to separate from a group in pairs or couples;
      specif. (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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pair \Pair\, v. t.
   1. 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.
      [1913 Webster]

            Glossy jet is paired with shining white. --Pope.
      [1913 Webster]

   2. To engage (one's self) with another of opposite opinions
      not to vote on a particular question or class of
      questions. [Parliamentary Cant]
      [1913 Webster]

   Paired fins. (Zool.) See under Fin.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pair
    n 1: a set of two similar things considered as a unit [syn:
         pair, brace]
    2: two items of the same kind [syn: couple, pair, twosome,
       twain, brace, span, yoke, couplet, distich,
       duo, duet, dyad, duad]
    3: two people considered as a unit
    4: a poker hand with 2 cards of the same value
    v 1: form a pair or pairs; "The two old friends paired off"
         [syn: pair, pair off, partner off, couple]
    2: bring two objects, ideas, or people together; "This fact is
       coupled to the other one"; "Matchmaker, can you match my
       daughter with a nice young man?"; "The student was paired
       with a partner for collaboration on the project" [syn:
       match, mate, couple, pair, twin]
    3: occur in pairs [syn: pair, geminate]
    4: arrange in pairs; "Pair these numbers" [syn: pair,
       geminate]
    5: engage in sexual intercourse; "Birds mate in the Spring"
       [syn: copulate, mate, pair, couple]

Moby Thesaurus II by Grady Ward, 1.0:

237 Moby Thesaurus words for "pair":
   Gaussian integer, accouple, accumulate, ace, affiliate,
   agglomerate, agglutinate, aggregate, aggroup, algebraic number,
   ally, amass, articulate, assemble, associate, band, band together,
   batch, be in cahoots, best bower, bond, both, bower, brace,
   bracket, bridge, bridge over, bring together, bulk, bunch,
   bunch together, bunch up, cabal, cardinal, cardinal number, cards,
   cement, cement a union, centralize, chain, clap together, club,
   club together, clubs, clump, cluster, collect, colligate,
   collocate, combine, come together, compare, compile,
   complex number, comprise, concatenate, confederate, conglobulate,
   conglomerate, conjoin, conjugate, connect, consociate, conspire,
   copulate, corral, couple, couple up, couplet, cover, craps,
   cumulate, deck, defective number, deuce, diamonds, dig up, distich,
   double, double harness, double-harness, double-team, doublet,
   doubleton, draw together, dredge up, drive together, duad, duet,
   dummy, duo, dyad, embrace, encompass, equipage, even number,
   face cards, federalize, federate, finite number, flush,
   four-in-hand, fraction, full house, gang, gang up, gather,
   gather in, gather together, get in, get together, glue,
   go in partners, go in partnership, group, hand, hearts,
   hook up with, imaginary number, impair, include, infinity, integer,
   irrational, irrational number, jack, join, join forces,
   join fortunes with, join together, join up with, join with, joker,
   juxtapose, king, knave, knot, lay together, league, left bower,
   link, look-alikes, lump together, make up, marry, marshal, mass,
   match, matching pair, mate, mates, merge, mixed number, mobilize,
   muster, ordinal, organize, pack, pair off, partner, picture cards,
   piece together, playing cards, polygonal number, prime number,
   pure imaginary, put together, queen, raise, rake up, rally, randem,
   rational, rational number, real, real number, rectangular number,
   rig, roll into one, round, round number, round up, royal flush,
   rubber, ruff, scrape together, serial number, set, set of two,
   singleton, snake eyes, solder, spades, span, spike, spike team,
   splice, stand together, stand up with, stick together, straight,
   surd, take in, take up, tandem, tape, team, team up, team up with,
   team with, the two, three-up, throw in with, tie, tie in with,
   tie up with, transcendental number, transfinite number, trey,
   trick, trump, turnout, twain, twin, twins, two, twosome, unicorn,
   unify, unionize, unite, unite with, wed, weld, whip in,
   whole number, yoke