Castration (also known as neutering or gonadectomy) is any action, surgical, chemical or otherwise, by which a biological male loses use of the testes. (Orchiectomy specifically refers to surgical removal of one or both testes.) This causes sterilization (i.e., prevents them from reproducing); it also greatly reduces the production of certain hormones, such as testosterone.
The term "castration" is sometimes also used to refer to the removal of the ovaries in the female, otherwise known as an oophorectomy or, in animals, spaying. Estrogen levels drop precipitously following oophorectomy, and long-term effects of the reduction of sex hormones are significant throughout the body.
Castration of non-human animals is intended for favouring a desired development of the animal or of his habits, or preventing overpopulation.