A tuber is a vegetable that grows underground, specifically it grows as a way for a plant to store energy for winter months. So, yams, all varieties of potato and cassava are the most common. These are full plants who make a, well, hibernation belly of starch underground to survive harsh times.

For reference, mushrooms are the only exception. There is plant type food (vegetables and fruits), animal type food (literally anything from an animal, meat, eggs, dairy, anything made from gelatin or marrow) and mushrooms. Since they are spores they have a different category. Tho that is just a scientific thing, like tomatoes and zucchinis are technically fruit. Mushrooms are considered a vegetable by all other sources (governments, nutritionists, medical doctors) since they provide the same sort of nutrition. So, if it’s not a fruit or mushroom it’s a vegetable. Or a well camouflaged chicken.