A bun is a small, usually sweet, bread. Commonly they are hand-sized or smaller, domed in shape, with a flat bottom. A bun can also be a savory bread roll similar to a bap or barmcake.

Basic buns are usually made using flour, sugar, milk, yeast, cardamom, and butter. Sometimes they contain sultanas and currants as well, and the iced bun is topped with an icing and sometimes filled with jam. Other fillings do occur. Varieties of buns do exist, such as round sticky buns, currant buns or iced buns.

"Bun" may also refer to a kind of filled dumpling, as Chinese baozi and the Lutonian Halapchi.

A bun can be long and skinny, short and round, and come in many different shapes and sizes.

In North England and Ireland, a bun is also a name given to small cakes, e.g. sponge buns, currant buns and butterfly buns.