yam1 /jam 
/ ▸ noun 1 the edible starchy tuber of a climbing plant that is widely grown in tropical and subtropical countries. 2 the cultivated plant that yields the yam. Genus Dioscorea, family Dioscoreaceae: many species.
3 North American English a sweet potato. – ORIGIN late 16th century: from Portuguese inhame or obsolete Spanish iñame, probably of West African origin.
EnglishOxford Dictionary of English
