The Collaborative International Dictionary of English v.0.48:

yummy \yum"my\ (y[u^]m"m[y^]), adj.
   Very pleasing or attractive; especially, pleasing to the
   taste; delicious; scrumptious. [colloq.]

   Syn: delectable, delicious, luscious, pleasant-tasting,
        scrumptious, toothsome.
        [PJC + WordNet 1.5]

WordNet (r) 3.0 (2006):

yummy
    adj 1: extremely pleasing to the sense of taste [syn:
           delectable, delicious, luscious, pleasant-
           tasting, scrumptious, toothsome, yummy]