From Longman Dictionary of Contemporary Englishsweet potatoˌsweet poˈtato noun [countable] 
DFa vegetable that looks like a red potato, is yellow inside, and tastes sweet → yam
DFa vegetable that looks like a red potato, is yellow inside, and tastes sweet → yamExamples from the Corpus
sweet potato• This treat he produced by mashing a sweet potato to pulp and adding sugar, vanilla extract, and evaporated milk.• The rest of the Brownies are sweet potato plants and they crouch in rows, making themselves as small as they can. 