mean time vs unit of time

mean time and unit of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
mean time	Yes	No	No	No
unit of time	Yes	No	No	No

As nouns, unit of time is a hypernym of mean time; that is, unit of time is a word with a broader meaning than mean time:

mean time: (astronomy) time based on the motion of the mean sun (an imaginary sun moving uniformly along the celestial equator)
unit of time: a unit for measuring time periods

Other hypernyms of mean time include time unit.

mean time (noun)	unit of time (noun)
(astronomy) time based on the motion of the mean sun (an imaginary sun moving uniformly along the celestial equator)	a unit for measuring time periods