A tuber is a plant that mainly grows underground. Potatoes and yams are tubers — and they're delicious with a little butter and salt.
The part of a potato plant that can be eaten is its thickened underground stem — and officially, that's the part of the plant considered a tuber. Potatoes are a stem tuber, while sweet potatoes are root tubers. There are slight differences in the way these different types of tubers grow new plants, but they're all basically edible roots. In Latin the word tuber means "edible root," but also "lump, bump, or swelling."