An orgasm is a sexual climax (or high point). An orgasm happens because of continued sexual touching. During an orgasm, there is a strong feeling of excitement and pleasure. Sexual touching may be sexual intercourse, rubbing or squeezing by hand (called masturbation), or other actions that make the sex organs feel good. 

During an orgasm, a person's body sometimes spasms (shakes or trembles), and the person may appear to be in pain or get a strange look on their face.

The person's brain activity changes quickly. The parts of the brain responsible for fear, worry and self-control become less active. Many people moan, shout or say strange things during orgasm. Usually, people's minds focus almost completely on the pleasure of the orgasm, and become less aware of what is going on around them than usual.

Orgasms may sometimes happen when people are asleep, this called a wet dream.

Other mammals may have orgasms when they have sex.

Male orgasm 
The penis usually becomes erect (hard) before an orgasm, and flaccid (soft) again after. When a man has an orgasm, his penis spasms and undergoes a series of rhythmic contractions, during which he feels a very strong and enjoyable feeling in his penis and groin, and sometimes in all of his body. In a man, an orgasm usually happens at the same time as an ejaculation, which is a release of semen through the penis that can't be stopped. Young boys can orgasm, but there may be no semen because semen release usually occurs (happens) only after puberty has started. After an orgasm, men usually have a deep sense of relaxation, usually felt in the groin and the thighs. Usually the stronger the orgasm is, the deeper the relaxation and the longer the relaxation will last.

Female orgasm
A woman's orgasm begins in her clitoris, which is the counterpart of the man's penis. It is preceded by an erection of the clitoris and a moistening of the opening of the vagina. When a woman goes through a full orgasm, her uterus, vagina, anus and pelvic muscles undergo a series of rhythmic contractions. Most women find these contractions very pleasurable. Rubbing or applying other consistent pressure against the clitoris is usually needed to help the woman have an orgasm.

Orgasm and reproduction
Orgasm plays an important role in human reproduction. It is the most exciting moment of sexual intercourse, so it is an important factor that makes men and women want to have intercourse, which allows reproduction to happen. It is also the moment at which the man releases semen into the woman to begin the process of fertilization.

During sexual intercourse, a man places his penis inside a woman's vagina and moves it backwards and forwards. The woman may also move her hips back and forth, or in a circular motion. The friction caused by this movement, together with the warmth and pressure of the vagina, causes the man to have pleasurable feelings in his penis. As intercourse continues, these feelings grow stronger and stronger until the man has an orgasm and ejaculates, at which time semen then shoots out from the end of his penis into the woman's vagina. The semen can make the woman pregnant.

The woman may also have one or more orgasms during intercourse as well. Unlike a man, who usually can have only one orgasm during intercourse, a woman may have more than one. During intercourse she may rub her clitoris, or she may have her man do so. This can make the woman have an orgasm. The shaft of the penis may also stimulate the clitoris in some positions.   Scientists are not sure why women have orgasms. Some believe that the orgasm helps the sperm in the man's semen to swim up the vagina into the cervix, so that it is more likely that a sperm cell will fuse with an egg cell. Others think that the woman's orgasm is designed to help make the man reach orgasm. During her orgasm, a woman may moan, cry, or scream, which usually excites her man, and her vagina may contract, gripping the penis more tightly and making sex more exciting for him. These reactions may cause the man to ejaculate more quickly or to produce more semen; when a woman reaches orgasm during intercourse, it often makes her man have an orgasm at that point as well. Orgasm may also encourage a woman to have sex more often as it feels good, increasing the chances of pregnancy.

Related pages
 List of sex positions

References

Further reading
Banker-Riskin, Anita; Grandinetti, Deborah (1997). Simultaneous Orgasm: And Other Joys of Sexual Intimacy. Hunter House. ISBN 0-89793-221-8, ISBN 978-0-89793-221-9.
PARTRIDGE, Eric (2001). Shakespeare's bawdy: Classics Series Routledge classics. 2nd ed., Routledge. ISBN 0-415-25400-0, ISBN 978-0-415-25400-7.
Plato (2001). The Banquet. (P.B. Shelley, Trans., J. Lauritsen, Ed., Foreword). Provincetown, MA: Pagan Press.
WEBB, Timothy (1976). The violet in the crucible: Shelley and translation, 1976. Oxford: Clarendon Press.
Gabriele Froböse, Rolf Froböse, Michael Gross (Translator): Lust and Love: Is it more than Chemistry? Publisher: Royal Society of Chemistry, ISBN 0-85404-867-7, (2006).
Komisaruk, Barry R.; Beyer-Flores, Carlos; Whipple, Beverly. The Science of Orgasm. Baltimore, MD; London: The Johns Hopkins University Press, 2006 (hardcover, ISBN 0-8018-8490-X).

Other websites

Men's Health: Male Orgasm
Net Doctor: Female Orgasm

Ted Talk by Mary Roach on 10 things you didn't know about orgasm

Sexuality