A cock ring is a ring that can be placed around a penis, usually at the base, primarily to slow the flow of blood from the erect penile tissue, thus maintaining an erection for a much longer period of time.
Cock rings can be worn around just the penis or both the penis and scrotum, or just the scrotum alone, though this is usually called a testicle cuff. Rings can be made of a variety of different materials, most commonly leather, rubber, or silicone, though nylon and metal are also used either as the main component or part of the closure.
The term cock ring is sometimes used as a synonym for a Prince Albert piercing, a piercing of the penis that is usually, though not always, a metal ring.
When used in cases of erectile dysfunction they are known by various names such as "erection ring" and "tension ring".