An orgasm is an involuntary and spontaneous physical reaction that generally (but not always) occurs in reaction to stimulation of the sexual organs - the penis in the male and the clitoris in the female. In the male, it is usually accompanied by ejaculation. It is generally characterized by a short loss of voluntary muscular control and reflex contractions or spasms, particularly in the muscles of the feet, the muscles in the pelvic region and the sphincter around the anus. It is also accompanied by overwhelming feelings of pleasure and euphoria.
Orgasm is generally due to deliberate action, but it is possible without any stimulation whatsoever. In adolescent males, this often manifests as ejaculation during sleep - nocturnal emission. Certain anti-depressants also cause spontaneous orgasms, in one case, every time a patient sneezed.