cornea vs arcus senilis

cornea and arcus senilis both are nouns.

Word	Noun	Adjective	Verb	Adverb
cornea	Yes	No	No	No
arcus senilis	Yes	No	No	No

As nouns, arcus senilis is a hyponym of cornea; that is, arcus senilis is a word with a more specific, narrower meaning than cornea:

cornea: the transparent dome-shaped anterior portion of the outer covering of the eye; it covers the iris and pupil and is continuous with the sclera
arcus senilis: a whitish deposit in the shape of an arc that is sometimes seen in the cornea

Other hyponyms of cornea include arcus.

cornea (noun)	arcus senilis (noun)
the transparent dome-shaped anterior portion of the outer covering of the eye; it covers the iris and pupil and is continuous with the sclera	a whitish deposit in the shape of an arc that is sometimes seen in the cornea