Ejaculation

Ejaculation is the process of ejecting semen from the penis, and is usually accompanied by orgasm as a result of sexual stimulation. It may also occur spontaneously during sleep (called a nocturnal emission), due to stimulation of the prostate or, rarely, due to prostatic disease.

For a more complete description read the wikipedia article on ejaculation.

Chastity devices such as the CB2000 are used by tops on their male slaves to prevent them from being able to reach orgasm