solar day vs mean solar day

solar day and mean solar day both are nouns.

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

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

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

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

solar day (noun)	mean 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