Why corn is a fruit, tomatoes are berries, rhubarb is a vegetable, and . The botanical definition of a berry. Check out the video below, where SciShow host Hank Green explains what makes a fruit a fruit, a vegetable a vegetable and a capsicum, confusingly, a berry. To a botanist, a fruit is an entity that develops from the fertilized ovary of a flower. This means that tomatoes, squash, pumpkins, cucumbers.