The Collaborative International Dictionary of English v.0.48:

Acetabulum \Ac`e*tab"u*lum\, n. [L., a little saucer for
   vinegar, fr. acetum vinegar, fr. acere to be sour.]
   [1913 Webster]
   1. (Rom. Antiq.) A vinegar cup; socket of the hip bone; a
      measure of about one eighth of a pint, etc.
      [1913 Webster]

   2. (Anat.)
      (a) The bony cup which receives the head of the thigh
          bone.
      (b) The cavity in which the leg of an insect is inserted
          at its articulation with the body.
      (c) A sucker of the sepia or cuttlefish and related
          animals.
      (d) The large posterior sucker of the leeches.
      (e) One of the lobes of the placenta in ruminating
          animals.
          [1913 Webster]

WordNet (r) 3.0 (2006):

acetabulum
    n 1: the cup-shaped hollow in the hipbone into which the head of
         the femur fits to form a ball-and-socket joint [syn:
         acetabulum, cotyloid cavity]