A true yam is an edible tuber from a plant species in the genus Dioscorea that is a vine, while a sweet potato is Ipomoea batatas, and I believe they are from a plant, not the root of a vine.
I believe that where yams actually are part of the diet, Africans brought from those areas as slaves thought the sweet potato resembled the proper yam, so called sweet potatoes by the same term.
Of course I'm told that a peanut is a legume, not a nut, although when one reads a label that says "does not contain nuts" it means peanuts as well as nuts from trees.
:idunno:
LD