Source : Webster's Revised Unabridged Dictionary (1913)

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.

   Note: Chaucer uses this word as equivalent to the siren of
         the ancients.

   {Mermaid fish} (Zo["o]l.) the angel fish ({Squatina}).

   {Mermaid's glove} (Zo["o]l.), a British branched sponge
      somewhat resembling a glove.

   {Mermaid's head} (Zo["o]l.), 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 {P.
      pectinacea}).

Source : WordNet®

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