A coat is an item of clothing.

Confusingly, what was once called a coat is now commonly referred to as a jacket, as in a dinner jacket, although the original meaning is still used in tailoring -- to mean the upper part of a suit, for example.

What is now commonly called a coat -- an outer layer of clothing such as a trench coat -- is, in tailoring terms, correctly called an overcoat, since it is worn over the coat.