noun
frontispiece; opening illustration
Refers to an illustration placed at the very beginning of a publication, before the main text. Common in books, magazines, and literary works.
この小説の口絵は有名な画家が描いたものです。
The frontispiece of this novel was drawn by a famous artist.
雑誌の口絵には美しい風景写真が使われている。
A beautiful landscape photo is used for the magazine's opening illustration.