mean solar day vs solar day

mean solar day and solar day both are nouns.

Word	Noun	Adjective	Verb	Adverb
mean solar day	Yes	No	No	No
solar day	Yes	No	No	No

As nouns, mean solar day and solar day are synonyms defined as:

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

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

mean solar day (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