How, I wondered, did this term come to have its present meaning, related as it is to capture (both coming from the Latin capere, to take. OED provided the answer.

The earliest senses are:

1. a. Taking, catching, seizure, capture. now rare.

1382 WYCLIF 2 Peter ii. 12 Beestes, kyndeli in to capcioun [Vulg. in captionem], or takinge

b. Law. Arrest or apprehension by judicial process. (esp. in Scotch law.)

1609 SKENE Reg. Maj. Table, 70 The forme of the breive of caption of ane debtour.

2. The action of cavilling or taking exception; an objection or cavil; fallacious or captious argument; a quibble, sophism. (L. captio.)

1605 BACON Adv. Learning II. xiv. §6. 55 The degenerate and corrupt vse is for Caption and contradiction.

3. Law. ‘That part of a legal instrument, as a commission, indictment, etc., which shows where, when, and by what authority it is taken, found, or executed’ (Tomlins Law Dict. 1809). This appears to be short for ‘certificate or note of caption or taking’; and it is sometimes used for the ‘making or execution’ of this certificate.

1670 BLOUNT Law Dict. s.v. Caption (Captio), When a Commission is executed, and the Commissioners names subscribed to a Certificate, declaring when and where the Commission was executed, that is called the Caption.

OED continues:

The foregoing is sometimes explained as ‘the beginning or heading of a warrant, commission, or indictment’, whence comes

4. The heading of a chapter, section, or newspaper article (chiefly used in U.S.). Also used (orig. U.S.) for the title below an illustration; in cinematography and television, a sub-title. Also attrib. and Comb.

1789 J. MADISON Writ. (1904) V. 355 You will see in the caption of the address that we have pruned the ordinary stile of the degrading appendages of Excellency, Esqrs. &c. 1821 Massachusetts Spy 24 Oct. (Th.), [The statute] is under the caption of ‘Fees in the Secretary’s office’. 1848 BARTLETT Dict. Amer., Caption: This legal term is used in the newspapers where an Englishman would say title, head, or heading. 1854 N. & Q. Ser. I. IX. 245/1 [A review] having three works as the caption of the article.

Interesting that Bartlett in the 1848 cite notes the sense as purely American. It clearly crossed the Atlantic soon after, as evidenced in the 1854 cite from the British periodical Notes & Queries.

Just a WAG, but it captures the essence of the thing that is captioned?

Edit:  Or I could read all of aldi’s post.

THE OED definition doesn’t properly fit (or at least it doesn’t seem to me to fit) the way “caption” is used in British newspapers and magazines, where we have a somewhat different usage to that found in the US - American usage, to quote this site is that

Captions are the little “headlines” over the “cutlines” (the words describing the photograph)… Cutlines (at newspapers and some magazines) are the words (under the caption, if there is one) describing the photograph or illustration.

In Britland, the caption is what appears under the picture, that is, what Americans call the “cutline”.

No, we don’t.

What you cite as “American usage” may be the technical terminology (or jargon, if you like) used in newspaper and magazine publishing, but it is not the general American usage.  I would venture to say that 99% of American readers are unfamiliar with the term “cutline” and the distinction between it and a caption.  In general American use, “caption” applies to the whole text appearing with a picture (almost always below,; if above the picture we might call it a title or headline).  For instance, see the AHD definition: “1. A title, short explanation, or description accompanying an illustration or a photograph.” In other words, “caption” means the same thing to a typical American as it means to you.

Zythophile - 11 July 2008 06:18 AM

In Britland, the caption is what appears under the picture, that is, what Americans call the “cutline”.

This American has never heard of a cutline.  I think it’s journalism jargon.  See the following from the same source:

Note:  Captions and cutlines are terms that are often used interchangeably, particularly at magazines, but not in this class. For our purposes, we will make the following distinctions.

Thank you, Dr T and Myridon, for making it clear that popular use is not the same as newspaper use in the US - I should have made it clearer myself that I was talking specifically about newspaper/magazine usage.

From another American who’s never heard of cutline:

I might refer to the text above an illustration as either a title or a caption. I’d be more inclined to use “caption” if it were consecutively numbered with the similar bits of text elsewhere in the document.

Underneath the illustration I’d always say caption.

My wife, the lovely AnnaS, knows the term “cutline” and can attest that it is still used in newspaper jargon today as it was several decades back when she was active in the field.

I don’t think anybody’s saying it’s not used in newspaper jargon, simply that that use is confined to jargon and that the vast majority of American speakers know the word only in the sense Dr. T describes.