testis

1. (medical) testicle; testis
   Lekar je pregledao testise i nije našao ništa. — The doctor examined the testicles and found nothing.
   Upala testisa zahteva hitan odlazak lekaru. — Inflammation of the testicle requires an urgent visit to a doctor.

This is a medical term, appropriate in neutral or medical discussion. In everyday speech people usually say "jaja" ("balls"), which is coarse informal slang and not for formal settings.