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's usually accompanied by ejaculation. It's 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's also accompanied by overwhelming feelings of pleasure and euphoria.
After an orgasm, the body releases prolactin and oxytocin, which both have a relaxing effect on the body.
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 antidepressants also cause spontaneous orgasms, in one case, every time a patient sneezed.
In Distractions, the patient had an orgasm while unconscious and while inside a hyperbaric chamber.
In Euphoria (Part 2), a young mother mistakes her young daughter's precocious masturbation as a seizure.