The Collaborative International Dictionary of English v.0.48:

Yam \Yam\ (y[a^]m), n. [Pg. inhame, probably from some native
   name.]
   1. (Bot.) A large, esculent, farinaceous tuber of various
      climbing plants of the genus Dioscorea; also, the plants
      themselves. Mostly natives of warm climates. The plants
      have netted-veined, petioled leaves, and pods with three
      broad wings. The commonest species is Dioscorea sativa,
      but several others are cultivated.
      [1913 Webster]

   2. (Bot.) Any one of several cultural varieties of the sweet
      potato. [U. S.]
      [Webster 1913 Suppl.]

   Chinese yam, a plant (Dioscorea Batatas) with a long and
      slender tuber, hardier than most of the other species.

   Wild yam.
      (a) A common plant (Dioscorea villosa) of the Eastern
          United States, having a hard and knotty rootstock.
      (b) An orchidaceous plant (Gastrodia sesamoides) of
          Australia and Tasmania.
          [1913 Webster]

WordNet (r) 3.0 (2006):

yam
    n 1: edible tuber of any of several yams
    2: any of a number of tropical vines of the genus Dioscorea many
       having edible tuberous roots [syn: yam, yam plant]
    3: sweet potato with deep orange flesh that remains moist when
       baked
    4: edible tuberous root of various yam plants of the genus
       Dioscorea grown in the tropics world-wide for food