The Collaborative International Dictionary of English v.0.48:

Waist \Waist\, n. [OE. wast; originally, growth, akin to AS.
   weaxan to grow; cf. AS. w[ae]stm growth. See Wax to grow.]
   [1913 Webster]
   1. That part of the human body which is immediately below the
      ribs or thorax; the small part of the body between the
      thorax and hips. --Chaucer.
      [1913 Webster]

            I am in the waist two yards about.    --Shak.
      [1913 Webster]

   2. Hence, the middle part of other bodies; especially
      (Naut.), that part of a vessel's deck, bulwarks, etc.,
      which is between the quarter-deck and the forecastle; the
      middle part of the ship.
      [1913 Webster]

   3. A garment, or part of a garment, which covers the body
      from the neck or shoulders to the waist line.
      [1913 Webster]

   4. A girdle or belt for the waist. [Obs.] --Shak.
      [1913 Webster]

   Waist anchor. See Sheet anchor, 1, in the Vocabulary.
      [1913 Webster]

WordNet (r) 3.0 (2006):

waist
    n 1: the narrowing of the body between the ribs and hips [syn:
         waist, waistline]
    2: the narrow part of the shoe connecting the heel and the wide
       part of the sole [syn: shank, waist]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "waist":
   blouse, bodice, center, core, corsage, diameter, diaphragm, dickey,
   equator, heart, interior, kernel, linen, mean, median, middle,
   midmost, midriff, midst, nucleus, shift, shirt, thick,
   thick of things, waistline, zone