A caption was text at the bottom of a photograph that identified its subject(s).
In 2369, the caption on a photograph of three Cardassians identified them as Aamin Marritza, Gul Darhe'el, and another man. (DS9: "Duet")
In 2380, when asked if she even read the mission brief on Tulgana IV, Beckett Mariner replied that she "skimmed almost most of it and read some of the captions". (LD: "Envoys")