solstice vs midsummer

solstice and midsummer both are nouns.

Word	Noun	Adjective	Verb	Adverb
solstice	Yes	No	No	No
midsummer	Yes	No	No	No

As nouns, midsummer is a hyponym of solstice; that is, midsummer is a word with a more specific, narrower meaning than solstice:

solstice: either of the two times of the year when the sun is at its greatest distance from the celestial equator
midsummer: June 21, when the sun is at its northernmost point

Other hyponyms of solstice include June 21, summer solstice, winter solstice.

solstice (noun)	midsummer (noun)
either of the two times of the year when the sun is at its greatest distance from the celestial equator	June 21, when the sun is at its northernmost point