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.