Final answer:
A retention cyst in the epididymis filled with fluid containing sperm is called a spermatocele, which is a benign condition.
Step-by-step explanation:
A retention cyst in the epididymis filled with milky fluid containing sperm is called a spermatocele. The epididymis is a coiled tubular structure that plays a crucial role in the maturation of spermatozoa. Sperm begin their development in the testes and then move to the epididymis, where they continue to mature and gain motility, crucial for fertilization. They are stored here until they are expelled from the body during ejaculation. When a cyst forms in the epididymis and is filled with a milky fluid that includes sperm, this is known as a spermatocele, which is generally benign.