Other forms: tarts; tarter; tartest
A tart is small pie filled with fruit or custard, with no top crust, like the cherry tarts you bought at the bakery.
As an adjective, tart describes a sour taste, like lemon, or harsh words, like your friend's tart reply to a question that makes her mad. In the 19th century, tart was British slang for "pretty woman." Some believe it is a shortening of "sweetheart." But by the end of that century, tart described a prostitute, something many language scholars trace back to the tart that you get at the bakery.