subtitle vs hardsub

subtitle and hardsub both are nouns.

Word	Noun	Adjective	Verb	Adverb
subtitle	Yes	No	Yes	No
hardsub	Yes	No	No	No

As nouns, hardsub is a hyponym of subtitle; that is, hardsub is a word with a more specific, narrower meaning than subtitle:

subtitle: translation of foreign dialogue of a movie or TV program; usually displayed at the bottom of the screen
hardsub: (often plural) a subtitle whose text is irreversibly merged in original video frames.

Other hyponyms of subtitle include hard subtitle.

subtitle (noun)	hardsub (noun)
secondary or explanatory title	(often plural) a subtitle whose text is irreversibly merged in original video frames.
translation of foreign dialogue of a movie or TV program; usually displayed at the bottom of the screen

subtitle (verb)	hardsub (verb)
supply (a movie) with subtitles