Cryptorchidism is the absence of one or both testes from the scrotum. The word is from the Greek κρυπτός, kryptos, meaning hidden, and ὄρχις, orchis, meaning testicle. It is the most common birth defect of the male genital tract. About 3% of full-term and 30% of premature infant boys are born with at least one undescended testis. However, about 80% of cryptorchid testes descend by the first year of life, making the true incidence of cryptorchidism around 1% overall. Cryptorchidism may develop after infancy, sometimes as late as young adulthood, but that is exceptional.