periapsis vs perihelion

periapsis and perihelion both are nouns.

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

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

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

Other hyponyms of periapsis include perigee, perijove, perilune, periselene.

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