mean solar day vs today

mean solar day and today both are nouns.

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

As nouns, today is a hyponym of mean solar day; that is, today is a word with a more specific, narrower meaning than mean solar day:

mean solar day: time for Earth to make a complete rotation on its axis
today: the day that includes the present moment (as opposed to yesterday or tomorrow)

Other hyponyms of mean solar day include tomorrow, yesterday, morrow, eve, date, day of the month, date, snow day.

mean solar day (noun)	today (noun)
time for Earth to make a complete rotation on its axis	the present time or age
	the day that includes the present moment (as opposed to yesterday or tomorrow)

mean solar day (adverb)	today (adverb)
	on this day as distinct from yesterday or tomorrow
	in these times