The Collaborative International Dictionary of English v.0.48:

collards \col"lards\ (k[o^]l"l[~e]rdz), n. pl. [Corrupted fr.
   colewort.]
   1. Young cabbage, used as "greens"; esp. the leaves of a kind
      of kale (Brassica oleracea acephala) cultivated for that
      purpose, the collard or colewort. [Colloq. Southern U. S.]

   Syn: collard greens.
        [1913 Webster +PJC]

   2. A kind of kale (Brassica oleracea acephala) cultivated
      in the southern United States as a vegetable.
      [PJC]

WordNet (r) 3.0 (2006):

collards
    n 1: kale that has smooth leaves [syn: collards, collard
         greens]