Source : Webster's Revised Unabridged Dictionary (1913)

Yam \Yam\ (y[a^]m), n. [Pg. inhame, probably from some native
   name.] (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 {D. sativa}, but several
   others are cultivated.

   {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.

Yam \Yam\, n. (Bot.)
   Any one of several cultural varieties of the sweet potato.
   [U. S.]

Source : WordNet®

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 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