Belles lettres in French is a name given to grammar, poetry, and literature in general. In English it means literature.

Originally (in French), they were an Adj plus a Noun, but now it's a Noun.

A belletrist is a person who writes or is concerned with belles-lettres.

The definition of belles-lettres is:

literature regarded as a fine art, especially as having a purely aesthetic function or light and elegant literature, especially that which is excessively refined, characterized by aestheticism, and minor in subject, substance, or scope.

Belles lettres in French is a name given to grammar, poetry, and literature in general. In English it means literature.