The Collaborative International Dictionary of English v.0.48:

Mermaid \Mer"maid\, n. [AS. mere lake, sea. See Mere lake, and
   maid.]
   A fabled marine creature, typically represented as having the
   upper part like that of a woman, and the lower like a fish; a
   sea nymph, sea woman, or woman fish.
   [1913 Webster]

   Note: Chaucer uses this word as equivalent to the siren of
         the ancients.
         [1913 Webster]

   Mermaid fish (Zool.) the angel fish (Squatina).

   Mermaid's glove (Zool.), a British branched sponge somewhat
      resembling a glove.

   Mermaid's head (Zool.), a European spatangoid sea urchin
      (Echinocardium cordatum) having some resemblance to a
      skull.

   Mermaid weed (Bot.), an aquatic herb with dentate or
      pectinate leaves (Proserpinaca palustris and
      Proserpinaca pectinacea).
      [1913 Webster]

WordNet (r) 3.0 (2006):

mermaid
    n 1: half woman and half fish; lives in the sea

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "mermaid":
   Davy, Davy Jones, Dylan, Neptune, Nereid, Nereus, Oceanid, Oceanus,
   Poseidon, Thetis, Triton, Varuna, bather, bathing beauty,
   bathing girl, diver, fresh-water nymph, frogman, kelpie, limniad,
   man fish, merman, naiad, natator, nix, nixie, ocean nymph,
   sea devil, sea god, sea nymph, sea-maid, sea-maiden, seaman, siren,
   swimmer, undine, water god, water spirit, water sprite