The Collaborative International Dictionary of English v.0.48:

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.
          [1913 Webster]

   2. A citadel; a fortress; hence, a defense.
      [1913 Webster]

            Thou hast been a shelter for me, and a strong tower
            from the enemy.                       --Ps. lxi. 3.
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   3. A headdress of a high or towerlike form, fashionable about
      the end of the seventeenth century and until 1715; also,
      any high headdress.
      [1913 Webster]

            Lay trains of amorous intrigues
            In towers, and curls, and periwigs.   --Hudibras.
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   4. High flight; elevation. [Obs.] --Johnson.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tower mustard
    n 1: or genus Arabis: erect cress widely distributed throughout
         Europe [syn: tower mustard, tower cress, Turritis
         glabra, Arabis glabra]
    2: European cress having stiff erect stems; sometimes placed in
       genus Turritis [syn: tower cress, tower mustard, Arabis
       turrita]