The Collaborative International Dictionary of English v.0.48:
Yam \Yam\ (y[a^]m), n. [Pg. inhame, probably from some native
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.
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.
(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.