Castration is any action, surgical, chemical, or otherwise, by which a male loses use of the testicles: the male gonad. Surgical castration is bilateral orchiectomy, while chemical castration uses pharmaceutical drugs to deactivate the testes. Castration causes sterilization ; it also greatly reduces the production of hormones, such as testosterone and estrogen. Surgical castration in animals is often called neutering.

Neutering, from the Latin neuter, is the removal of a non-human animal's reproductive organ, either all of it or a considerably large part. The male-specific term is castration, while spaying is usually reserved for female animals. Colloquially, both terms are often referred to as fixing. In male horses, castrating is referred to as gelding. An animal that has not been neutered is sometimes referred to as entire or intact.