Source : Webster's Revised Unabridged Dictionary (1913)

Tower \Tow"er\, n. [OE. tour,tor,tur, F. tour, L. turris; akin
   to Gr. ?; cf. W. twr a tower, Ir. tor a castle, Gael. torr a
   tower, castle. Cf. {Tor}, {Turret}.]
   1. (Arch.)
      (a) A mass of building standing alone and insulated,
          usually higher than its diameter, but when of great
          size not always of that proportion.
      (b) A projection from a line of wall, as a fortification,
          for purposes of defense, as a flanker, either or the
          same height as the curtain wall or higher.
      (c) A structure appended to a larger edifice for a special
          purpose, as for a belfry, and then usually high in
          proportion to its width and to the height of the rest
          of the edifice; as, a church tower.

   2. A citadel; a fortress; hence, a defense.

            Thou hast been a shelter for me, and a strong tower
            from the enemy.                       --Ps. lxi. 3.

   3. A headdress of a high or towerlike form, fashionable about
      the end of the seventeenth century and until 1715; also,
      any high headdress.

            Lay trains of amorous intrigues In towers, and
            curls, and periwigs.                  --Hudibras.

   4. High flight; elevation. [Obs.] --Johnson.

   {Gay Lussac's tower} (Chem.), a large tower or chamber used
      in the sulphuric acid process, to absorb (by means of
      concentrated acid) the spent nitrous fumes that they may
      be returned to the Glover's tower to be reemployed. See
      {Sulphuric acid}, under {Sulphuric}, and {Glover's tower},
      below.

   {Glover's tower} (Chem.), a large tower or chamber used in
      the manufacture of sulphuric acid, to condense the crude
      acid and to deliver concentrated acid charged with nitrous
      fumes. These fumes, as a catalytic, effect the conversion
      of sulphurous to sulphuric acid. See {Sulphuric acid},
      under {Sulphuric}, and {Gay Lussac's tower}, above.

   {Round tower}. See under {Round}, a.

   {Shot tower}. See under {Shot}.

   {Tower bastion} (Fort.), a bastion of masonry, often with
      chambers beneath, built at an angle of the interior
      polygon of some works.

   {Tower mustard} (Bot.), the cruciferous plant {Arabis
      perfoliata}.

   {Tower of London}, a collection of buildings in the eastern
      part of London, formerly containing a state prison, and
      now used as an arsenal and repository of various objects
      of public interest.

Source : WordNet®

Tower of London
     n : a fortress in London on the Thames; used as a palace and a
         state prison and now as a museum containing the crown
         jewels