General Synonyms:
* Caption
* Legend
* Title
* Heading
* Label
* Tag
* Designation
* Supertitle
More specific synonyms:
* Closed captioning (for subtitles that are displayed on a screen)
* Subtitles for the deaf and hard of hearing (for subtitles specifically designed for accessibility)
* Translation (for subtitles that translate the dialogue into a different language)
* Intertitles (for subtitles used in silent films)
The best synonym for "subtitle" will depend on the specific context in which it is used.