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.]
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   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.]
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               Two crowns in my pocket, two pair of cards.
                                                  --Beau. & Fl.
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   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.
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   3. Two of a sort; a span; a yoke; a couple; a brace; as, a
      pair of horses; a pair of oxen.
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   4. A married couple; a man and wife. "A happy pair."
      --Dryden. "The hapless pair." --Milton.
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   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.
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   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.
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   7. (Kinematics) In a mechanism, two elements, or bodies,
      which are so applied to each other as to mutually
      constrain relative motion.
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   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.
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   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.]
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   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."
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