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
see also:
mere maid mermaid fish squatina mermaid's glove mermaid's head 
echinocardium cordatum mermaid weed proserpinaca palustris proserpinaca pectinacea