Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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