1. A brief description accompanying an illustration.
2. Translation of foreign dialogue for a movie or TV program; usually displayed at the bottom of the screen; such as, a printed explanation in a motion picture or on television; especially, a translation of dialogue accompanying a scene or an explanation preceding a scene.
3. To provide with a caption; for example, a photograph or a drawing.
4. A heading or subheading in a document or article.
5. An attachment to or heading of a legal document that identifies the circumstances of its production and the sources of its authority.
6. Etymology: from about A.D. 1384, "taking, seizure", from Old French capcion, from Latin capito; past participle of capere "to take".

Its more recent meanings evolved from headings of legal indictments involving seizure ("Certificate of caption"); then the meaning, "the beginning of a document" and "the heading of a chapter or section of an article" has evolved; especially, in the U.S., to "a description or title below an illustration".