undescended testicle vs undescended testis

undescended testicle and undescended testis both are nouns.

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

As nouns, undescended testicle and undescended testis are synonyms defined as:

undescended testicle and undescended testis: a testis that fails to move into the scrotum as the male fetus develops

undescended testicle (noun)	undescended testis (noun)
a testis that fails to move into the scrotum as the male fetus develops	a testis that fails to move into the scrotum as the male fetus develops