1. An exterior angle of a wall or other piece of masonry.
2. Any of the stones used in forming such an angle, often being of large size and dressed or arranged so as to form a decorative contrast with the adjoining walls.
3. A stone block used to form a quoin, especially when it is different; such as, in size, texture, or material, from the other blocks or bricks that are used to build the wall.
4. Etymology: "a cornerstone", a variant of coin; originally, in other senses of that word, too, including "a wedge"; from Old French coing (12th century), "a wedge; stamp; piece of money; corner, angle"; from Latin cuneus, "a wedge".

The die for stamping metal was wedge-shaped, and the English word came to mean "thing stamped, a piece of money" by the late 14th century.

In Modern French, coin is "a corner, an angle", or "a nook" (interior angle formed between two meeting walls or a small corner or sheltered space).