The Collaborative International Dictionary of English v.0.48:

Tap \Tap\, n. [AS. t[ae]ppa, akin to D. tap, G. zapfen, OHG.
   zapfo, Dan. tap, Sw. tapp, Icel. tappi. Cf. Tampion,
   Tip.]
   1. A hole or pipe through which liquor is drawn.
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   2. A plug or spile for stopping a hole pierced in a cask, or
      the like; a faucet.
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   3. Liquor drawn through a tap; hence, a certain kind or
      quality of liquor; as, a liquor of the same tap. [Colloq.]
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   4. A place where liquor is drawn for drinking; a taproom; a
      bar. [Colloq.]
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   5. (Mech.) A tool for forming an internal screw, as in a nut,
      consisting of a hardened steel male screw grooved
      longitudinally so as to have cutting edges.
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   On tap.
      (a) Ready to be drawn; as, ale on tap.
      (b) Broached, or furnished with a tap; as, a barrel on
          tap.

   Plug tap (Mech.), a screw-cutting tap with a slightly
      tapering end.

   Tap bolt, a bolt with a head on one end and a thread on the
      other end, to be screwed into some fixed part, instead of
      passing through the part and receiving a nut. See Illust.
      under Bolt.

   Tap cinder (Metal.), the slag of a puddling furnace.
      [1913 Webster]

WordNet (r) 3.0 (2006):

on tap
    adj 1: (of liquids) available by turning a tap; "ale on tap";
           "hot and cold running water on tap"
    2: available for immediate use; "extra personnel on tap"; "other
       sports and entertainment facilities are on tap"; "there are
       numerous projects on tap"