winter solstice vs solstice

winter solstice and solstice both are nouns.

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

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

winter solstice: December 22, when the sun is at its southernmost point
solstice: either of the two times of the year when the sun is at its greatest distance from the celestial equator

winter solstice (noun)	solstice (noun)
December 22, when the sun is at its southernmost point	either of the two times of the year when the sun is at its greatest distance from the celestial equator