sweet potato vs yam

sweet potato

noun
  • A tropical perennial American vine, Ipomoea batatas, having a fleshy tuber. 

  • The tuber of this plant cooked as a vegetable. 

  • An ocarina. 

yam

noun
  • A sweet potato; a tuber from the species Ipomoea batatas. 

  • An orange-brown colour, like the flesh of the yam. 

  • A potato. 

  • home 

  • The edible, starchy, tuberous root of that plant, a tropical staple food. 

  • Any climbing vine of the genus Dioscorea in the Eastern and Western hemispheres, usually cultivated. 

  • A oca; a tuber from the species Oxalis tuberosa. 

  • Taro. 

verb
  • To eat. 

How often have the words sweet potato and yam occurred in a corpus of books? (source: Google Ngram Viewer )