undescended testis vs testicle

undescended testis and testicle both are nouns.

Word	Noun	Adjective	Verb	Adverb
undescended testis	Yes	No	No	No
testicle	Yes	No	No	No

As nouns, testicle is a hypernym of undescended testis; that is, testicle is a word with a broader meaning than undescended testis:

undescended testis: a testis that fails to move into the scrotum as the male fetus develops
testicle: one of the two male reproductive glands that produce spermatozoa and secrete androgens

Other hypernyms of undescended testis include ball, ballock, bollock, egg, nut, orchis, testis.

undescended testis (noun)	testicle (noun)
a testis that fails to move into the scrotum as the male fetus develops	one of the two male reproductive glands that produce spermatozoa and secrete androgens