The Collaborative International Dictionary of English v.0.48:

Calla \Cal"la\ (k[a^]l"l[.a]), n. [Linn[ae]us derived Calla fr.
   Gr. ??????? a cock's wattles but cf. L. calla, calsa, name of
   an unknown plant, and Gr. kalo`s beautiful.] (Bot.)
   A genus of plants, of the order Arace[ae].
   [1913 Webster]

   Note: The common Calla of cultivation is Richardia
         Africana, belonging to another genus of the same
         order. Its large spathe is pure white, surrounding a
         fleshy spike, which is covered with minute apetalous
         flowers.
         [1913 Webster]

WordNet (r) 3.0 (2006):

calla
    n 1: South African plant widely cultivated for its showy pure
         white spathe and yellow spadix [syn: calla lily, calla,
         arum lily, Zantedeschia aethiopica]
    2: water arum [syn: Calla, genus Calla]