Hydrocele – Wikipedia

A hydrocele is an accumulation of serous fluid in a body cavity. A hydrocele testis is the accumulation of fluids around a testicle. It is often caused by peritoneum wrapped around the testicle, called the tunica vaginalis. Provided there is no hernia present, it goes away without treatment in the first year.

https://en.wikipedia.org/wiki/Hydrocele