The Collaborative International Dictionary of English v.0.48:

Physa \Phy"sa\, prop. n. [NL., fr. Gr. fy^sa a bellows.] (Zool.)
   A genus of fresh-water Pulmonifera, having reversed spiral
   shells. See Pond snail, under Pond.
   [1913 Webster]

WordNet (r) 3.0 (2006):

physa
    n 1: any member of the genus Physa