Neutering, from the Latin neuter ('of neither sex'), is the removal of an 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.

Neutering is the most common method for animal sterilization. Humane societies, animal shelters, and rescue groups urge pet owners to have their pets neutered to prevent the births of unwanted litters, which contribute to the overpopulation of unwanted animals in the rescue system. Many countries require that all adopted cats and dogs be sterilized before going to their new homes.