Testicular cancer: A malignant tumor of the male sex organ (testicle) that normally produces the hormone testosterone. It is one of the most common cancers in young men. Most testicular cancers are found by men themselves, as lumps in the testicles. The risk of testicular cancer is increased in males whose testicles did not move down normally into the scrotum during childhood (undescended testicles). When a growth in a testicle is detected, cancer is confirmed after surgical removal of the affected testicle (orchiectomy) and examination of the tissue under a microscope. Testicular cancer is almost always curable if it is found early.