The Collaborative International Dictionary of English v.0.48:

Pool \Pool\, n. [F. poule, properly, a hen. See Pullet.]
   [Written also poule.]
   1. The stake played for in certain games of cards, billiards,
      etc.; an aggregated stake to which each player has
      contributed a snare; also, the receptacle for the stakes.
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   2. A game at billiards, in which each of the players stakes a
      certain sum, the winner taking the whole; also, in public
      billiard rooms, a game in which the loser pays the
      entrance fee for all who engage in the game; a game of
      skill in pocketing the balls on a pool table.
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   Note: This game is played variously, but commonly with
         fifteen balls, besides one cue ball, the contest being
         to drive the most balls into the pockets.
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               He plays pool at the billiard houses.
                                                  --Thackeray.
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   3. In rifle shooting, a contest in which each competitor pays
      a certain sum for every shot he makes, the net proceeds
      being divided among the winners.
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   4. Any gambling or commercial venture in which several
      persons join.
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   5. A combination of persons contributing money to be used for
      the purpose of increasing or depressing the market price
      of stocks, grain, or other commodities; also, the
      aggregate of the sums so contributed; as, the pool took
      all the wheat offered below the limit; he put $10,000 into
      the pool.
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   6. (Railroads) A mutual arrangement between competing lines,
      by which the receipts of all are aggregated, and then
      distributed pro rata according to agreement.
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   7. (Law) An aggregation of properties or rights, belonging to
      different people in a community, in a common fund, to be
      charged with common liabilities.
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   Pin pool, a variety of the game of billiards in which small
      wooden pins are set up to be knocked down by the balls.

   Pool ball, one of the colored ivory balls used in playing
      the game at billiards called pool.

   Pool snipe (Zool.), the European redshank. [Prov. Eng.]

   Pool table, a billiard table with pockets.
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WordNet (r) 3.0 (2006):

pool ball
    n 1: ball used in playing pool