The Collaborative International Dictionary of English v.0.48:

Water arum \Wa"ter a"rum\ (Bot.)
   An aroid herb (Calla palustris) having a white spathe. It
   is an inhabitant of the north temperate zone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Calla palustris
    n 1: plant of wetlands and bogs of temperate regions having
         small greenish flowers partly enclosed in a white spathe
         and red berries [syn: wild calla, water arum, Calla
         palustris]