twenty-four hours vs solar day

twenty-four hours and solar day both are nouns.

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

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

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

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

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