What is a "sour cherry"?

The sour cherry, also known as tart cherry or pie cherry, is a type of cherry known for its distinctively tart flavor. Smaller and firmer than sweet cherries, it has a bright to dark red skin when ripe, and typically has a soft flesh surrounding a pit or stone in the center. Sour cherries are commonly used in cooking and baking to add tartness and flavor to various dishes, including pies, jams, sauces, and desserts. They are also used to make beverages such as cherry juice and cherry liqueur.