perihelion vs periapsis

perihelion and periapsis both are nouns.

Word	Noun	Adjective	Verb	Adverb
perihelion	Yes	No	No	No
periapsis	Yes	No	No	No

As nouns, periapsis is a hypernym of perihelion; that is, periapsis is a word with a broader meaning than perihelion:

perihelion: periapsis in solar orbit; the point in the orbit of a planet or comet where it is nearest to the sun
periapsis: (astronomy) the point in an orbit closest to the body being orbited

Other hypernyms of perihelion include point of periapsis.

perihelion (noun)	periapsis (noun)
periapsis in solar orbit; the point in the orbit of a planet or comet where it is nearest to the sun	(astronomy) the point in an orbit closest to the body being orbited