Final answer:
A neoplasm of the germ cells of the testis is called a germ cell tumor, which can lead to testicular cancer, common in males aged 15 to 35.
Step-by-step explanation:
The term that means "a neoplasm of the germ cells of the testis" is a germ cell tumor. These tumors arise from the least mature cells in the testes known as spermatogonia, which are the stem cells responsible for the process of spermatogenesis. All stages of developing sperm are surrounded by Sertoli cells, which help in the maturation and production of sperm. If these germ cells begin to grow out of control, it can result in the formation of a testicular cancer, which is the most common in males aged 15 to 35 and can often be cured with early detection and surgery.