A cage made of plastic, silicone, metal, or another material that fully encompasses a penis. The cage is then locked so it can't be taken off without the key. It's designed to prevent erections & masturbation and is often used in
bdsm relationships where the key is given to someone's significant other, called a keyholder. It is still possible to
masturbate in them however, either by using a
vibrator on the cage or using a dildo in the ass.