From Spanking Art

A caption is a short piece of narrating text that is written under a picture, chart, or table. (Less commonly, a caption may appear above the image to which it is attached.)

Captions are found in illustrations, cartoons, caricatures, comics, and manga.

In comics, captions are usually enclosed in rectangular boxes and positioned close to the upper or lower panel frame. In colored comics, caption boxes are often filled with a non-white background color to make it easier for the reader to distinguish them from balloons (which usually have white backgrounds).