The Collaborative International Dictionary of English v.0.48:

Planaria \Pla*na"ri*a\, n.; pl. L. Planari[ae], E. -rias.
   [NL. See Planary.] (Zool.)
   Any species of turbellarian worms belonging to Planaria,
   and many allied genera. The body is usually flat, thin, and
   smooth. Some species, in warm countries, are terrestrial.
   [1913 Webster]

WordNet (r) 3.0 (2006):

planaria
    n 1: free-swimming mostly freshwater flatworms; popular in
         laboratory studies for the ability to regenerate lost parts
         [syn: planarian, planaria]