Source : Webster's Revised Unabridged Dictionary (1913)

Pair \Pair\, n. [F. paire, LL. paria, L. paria, pl. of par pair,
   fr. par, adj., equal. Cf. {Apparel}, {Par} equality, {Peer}
   an equal.]
   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, except as to stairs.]

               Two crowns in my pocket, two pair of cards.
                                                  --Beau. & Fl.

   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.

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

   4. A married couple; a man and wife. ``A happy pair.''
      --Dryden. ``The hapless pair.'' --Milton.

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

   6. Two members of opposite parties or opinion, as in a
      parliamentary body, who mutually agree not to vote on a
      given question, or on issues of a party nature during a
      specified time; as, there were two pairs on the final
      vote. [Parliamentary Cant]

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

   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.

   {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}.]

   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.''