summer solstice vs solstice

summer solstice and solstice both are nouns.

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

As nouns, solstice is a hypernym of summer solstice; that is, solstice is a word with a broader meaning than summer solstice:

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

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