solar day vs today

solar day and today both are nouns.

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

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

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

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)

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