An orgasm, also known as a sexual climax, is a pleasurable psychological or emotional response to prolonged sexual stimulation. It is often accompanied by a notable physiological reaction, such as ejaculation, blushing or spasm.
Both males and females can experience orgasm, but the exact response varies across sex. Generally speaking, orgasm is the third stage of four in the human sexual response cycle, which is the currently accepted model of the physiological process of sexual stimulation. It is usually thought of as being the most rewarding part of having sex, even though a sizable minority reports liking sex for its other qualities.
Orgasm is the conclusion of the plateau phase of the sexual response cycle, shared by males and females alike. During orgasm, both males and females experience quick cycles of muscle contraction in the lower pelvic muscles, which surround both the anus and the primary sexual organs.
Orgasms in both men and women are often associated with other involuntary actions, including vocalizations and muscular spasms in other areas of the body. Also, a generally euphoric sensation is associated with orgasm.
Afterwards, orgasm generally causes perceived tiredness, and both males and females often feel a need to rest. This is often attributed to the release of endorphins during orgasm causing relaxation and drowsiness, but can also be due to the body's need for a short rest after a bout of vigorous physical activity.
Human male orgasm
In a human male orgasm, there are rapid, rhythmic contractions of the prostate, urethra and the muscles at the base of the penis, which force stored semen to be expelled through the penis's urethral opening. This is referred to as ejaculation. The process usually takes from 3 to 10 seconds. The process is usually, but not always, extremely pleasurable. Orgasm is generally induced by direct stimulation of the penis. Some men experience heightened orgasm by direct stimulation of the prostate through the perineum, or with a finger or dildo inserted in the anus.
Note that it is possible to have an orgasm without ejaculation, dry orgasm, or to ejaculate without reaching orgasm. Some men report that the ability to consciously separate orgasm and ejaculation has allowed them to achieve multiple orgasms.
Following ejaculation, a refractory period occurs during which a man cannot have another orgasm. This period can be anywhere from less than a minute to over half a day, depending on age and other individual factors. A very few cases have been reported of men who appear to have no refractory period at all.
Post-operative female-to-male transsexual men (after having undergone metoidioplasty or phalloplasty by a qualified surgeon) generally experience orgasm in the same way, except that those who have had phalloplasty have a pump installed to create an erection, as the neopenis (with either surgical technique) has limited natural erection capability.
Human female orgasm
In a human female orgasm, orgasm is preceded by moistening of the vaginal walls, and an enlargement of the clitoris due to increased blood flow trapped in the clitoris's spongy tissue. Some women exhibit a sex flush; a reddening of the skin over much of the body due to increased blood flow to the skin. As a woman comes closer to having orgasm, the clitoris moves inward under the clitoral hood, and the labia minora (minor lips) become darker. As orgasm becomes imminent, the vagina decreases in size by about 30% and also becomes congested with blood. The uterus then experiences muscular contractions. A woman experiences full orgasm when her uterus, vagina and pelvic muscles undergo a series of rhythmic contractions.
After the orgasm is over, the clitoris re-emerges from under the clitoral hood, and returns to its normal size in less than 10 minutes. Unlike men, women do not have a refractory period, and thus can experience a second orgasm soon after the first; some women can even follow this with a third, or even fourth orgasm; this is known as multiple orgasms. Research shows that about 13% of women experience multiple orgasms; a larger number may be able to experience this with the proper stimulation (such as a vibrator) and frame of mind. However, some women's clitorises are too sensitive after orgasm, making additional stimulation painful; they are probably not able to experience multiple orgasms.
Post-operative male-to-female transsexual women (having undergone vaginoplasty by a qualified surgeon) generally experience full orgasm, involving any combination of the clitoris, vagina and labia.
Debate rages over the "purpose" of female orgasms. Many different theories have been advanced by different scientists, feminists, doctors, and religious groups.
Some evolutionary biologists believe that female orgasms have a distinct purpose, such as increasing intimacy with a male partner in order to ensure the survival of the pair bond. Others have theorized that they increase fertility by enhancing sperm retention.
The clitoris is homologous to the penis, that is, it develops similarly to the penis in the embryo, until female hormones alter its development. It has been claimed by some researchers, such as Stephen Jay Gould that the clitoris is vestigial in the female, and that female orgasm serves no particular function. Proponents of this theory, such as Dr. Elisabeth Lloyd, point to the relative difficulty of achieving female orgasm through vaginal sex, and limited evidence for increased fertility after orgasm. Feminists such as Natalie Angier have criticized this theory, because it understates the value of female orgasm.
Some women experience an expulsion of fluid during orgasm. In many cases the origin of this fluid is the Skene's glands. In transsexual women, this can be from remnants of the prostate gland, which is not removed during vaginoplasty. For further details, see female ejaculation.
Vaginal vs. clitoral orgasms
A distinction is sometimes made between clitoral and vaginal orgasms in women. An orgasm that results from combined clitoral and vaginal stimulation is called a blended orgasm. Many doctors and feminist advocates have claimed that vaginal orgasms do not exist, and that female orgasms are obtained only from clitoral arousal. Recent discoveries about the size of the clitoris — it extends inside the body, around the vagina — would seem to support this theory. On the other hand, other sources argue that vaginal orgasms are dominant or more "mature."
This latter viewpoint was first promulgated by Sigmund Freud. In 1905, Freud argued that clitoral orgasm was an adolescent phenomenon, and upon reaching puberty the proper response of mature women changes to vaginal orgasms.  (http://www.uno.edu/~asoble/pages/koedt.htm) While Freud did not provide evidence supporting this basic assumption, the consequences of the theory were greatly elaborated thereafter.
In 1966, Masters and Johnson published pivotal research into the phases of sexual stimulation. Their work included women as well as men, and unlike Kinsey previously (in 1948 and 1953), set out to determine the physiological stages leading up to and following orgasm.  (http://health.discovery.com/centers/sex/sexpedia/mandj.html) One of the results was the promotion of the idea that vaginal and clitoral orgasms follow the same stages of physical response. Additionally, Masters and Johnson argued that clitoral stimulation is the primary source of orgasms.
This standpoint has been adopted by feminist advocates, to the extent that some hold that the vaginal orgasm was a mirage, created by men for their convenience. Certainly many women can only experience orgasm with clitoral stimulation, either alone or in addition to vaginal stimulation, while (less commonly) other women can only experience orgasm with vaginal stimulation. The clitoral-only orgasm school of thought became an article of faith in some feminist circles. Alternatively, some feminists instead feel the clitoral orgasm robs females of the source of the womanhood.
A new understanding of vaginal orgasm has been emerging since the 1980s. Many women report that some form of vaginal stimulation is essential to subjectively experience a complete orgasm, in addition to or in lieu of external (clitoral) stimulation. Recent anatomical research has pointed towards a connection between intravaginal tissues and the clitoris. It has been shown that these tissues have connecting nerves. This, combined with the anatomical evidence that the internal part of the clitoris is a much larger organ than previously thought could also explain credible reports of orgasms in women who have undergone clitoridectomy as part of so-called female circumcision.
In some cases it is possible for women to orgasm through stimulation of secondary sexual organs (eg breasts), and in very rare cases, without any direct stimulation to the genitalia or the other specific erogenous zones, but instead stimulation of the non-specific zones (i.e. neck).
The inability to have orgasm is called anorgasmia, or inorgasmia. In situations where orgasm is desired, anorgasmia is mainly thought of as being caused by an inability to relax, or 'let go'. It seems to be tightly associated with performance pressure in intercourse, and an unwillingness to pursue pleasure as such, as separate from the other person's satisfaction.
For a variety of reasons, some people choose to fake an orgasm.
Orgasm in non-humans
The mechanics of the male orgasm are similar in most mammals. However, it is questionable whether other animals experience orgasm in ways similar to humans.
There is some evidence that some non-human animals, particularly primates, can experience orgasm in ways similar to humans.
- Singer, J., and I. Singer. Types of Female Orgasm. In J. LoPiccolo and L. LoPiccolo, eds., Handbook of Sex Therapy. New York: Plenum Press, 1978.