In the United States, at least, yam and sweet potato are often used interchangeably to describe the root vegetable with moist, orange flesh that looks like this:
images downloaded from Pixabay
Unless you live near a West African or Caribbean grocery, however, you have probably never eaten an actual yam. Yams and sweet potatoes are distinct species of plant—in fact, they don’t even belong to the same basic category of plant.
Furthermore, yams are not roots. The flesh of a sweet potato consists of the carbohydrates the plant has stored in root form, but a yam is an underground stem, or tuber. (Some sources distinguish between stem tubers and root tubers—according to these sources, sweet potatoes are also tubers. Regardless, sweet potatoes are derived from roots and yams are derived from stems.)
Compared to the familiar, American sweet potato, yams—which are native to Africa and Asia—are drier and starchier, with rougher skin. Yam flesh comes in a variety of colors, from off-white to pink or purple. (For that matter, sweet potatoes are also different colors; some have yellow rather than orange flesh.) As seen in the photograph at right, yams can grow quite large.
image downloaded from Wikimedia Commons
This page lists additional differences between yams and sweet potatoes.
Why do some people call sweet potatoes yams?
I’ve encountered several theories, but the likeliest explanation—supported by writers for the Smithsonian and Library of Congress—dates back several centuries, to when Americans were mainly familiar with yellow-fleshed sweet potatoes. The West African word for the food we call yam is nyami (or at least it is similar to nyami—I haven’t found a definitive source). When the orange-fleshed sweet potato was introduced to U.S. markets, African slaves called the vegetables nyami. English speakers shortened nyami to yam, and the new word stuck—it proved a useful way to distinguish orange-fleshed sweet potatoes from the yellow-fleshed variety. Today, sweet potatoes are labeled “yams” in many stores, though the U.S. Department of Agriculture requires such labels also to note the vegetable’s true identity as a sweet potato. (Multiple sources emphasize this last detail, though I couldn’t find a USDA webpage to confirm it.)