[jbri]

There are sort of two different definitions at work here - the semantic definition which is as you describe, and then there's the technical definition which has to do with the implementation.

"Subtitles" on a DVD are basically a 4-colour (3 + transparent) bitmap which is overlaid on the video stream. But DVDs also support the closed-captioning that was originally used for TV broadcasts, which is text embedded in the video signal that's decoded and displayed by the television set itself.

Media playing software can extract subtitles from the latter system and display them in whatever font you like - but you can't do the same with the DVD-overlay subtitles, because they're just images.