mean solar day vs twenty-four hours

mean solar day and twenty-four hours both are nouns.

Word	Noun	Adjective	Verb	Adverb
mean solar day	Yes	No	No	No
twenty-four hours	Yes	No	No	No

As nouns, mean solar day and twenty-four hours are synonyms defined as:

mean solar day and twenty-four hours: time for Earth to make a complete rotation on its axis

Other synonyms of mean solar day include 24-hour interval, day, solar day, twenty-four hour period.

mean solar day (noun)	twenty-four hours (noun)
time for Earth to make a complete rotation on its axis	time for Earth to make a complete rotation on its axis