Where one injects a stream of liquid into the anus, usually with the purpose of cleaning the tract of fecal matter. Also used for self pleasure, of feeling the warm sensation of liquid within the anus.
Douche comes from the French, meaning shower.
Some medical experts consider it a risk, though no actual evidence exists to substantiate the claim.
While the primary purpose is for cleaning the rectal region of fecal matter, there are those who simply enjoy the sensation of defecating, which this accomplishes.
Additionally, it can involve the pleasure/thrill of attempting to hold back such actions, that is the goal.
Mainly it is used as means of hygiene, to remove any fecal matter within the body, prior to engaging in anal sex, or fisting.
Can also be used as a form of humiliation during BDSM sex games, or other sex games.
Urine has been known to be used, as the liquid, as part of the sexual thrill, though it carries a risk of infection.
Most commonly used is warm water, a rubber hose, with a bulb attachment to create the pressure, that directs the stream of warm liquid into the subject, from the inserted hose.
There are even sex novelties, that resemble a penis, that have the ability to inject a liquid to simulate ejaculation. Mainly used in masturbation or safer sex game rituals.