A plum is a flowering fruit tree, and it's also the name of the fruit that grows on it. Plums are small, dark reddish purple, and sweet.
Plums have an edible, smooth skin, juicy flesh, and a hard pit in the middle. When plums are dried, they're called prunes. The typical color of a ripe plum — a rich, purple-red shade — is also called plum. And informally, you can say that something great or desirable is plum: "I was just offered a plum role in the school play!"