Ejaculation is the process of ejecting semen from the penis. This usually occurs due to sexual stimulation, but may also occur spontaneously during sleep, and then is called a nocturnal emission, or may occur due to massaging of the prostate or, rarely, due to prostatic disease.
Ejaculation as a process usually is caused by a combination of contractions of the prostate, a muscular gland, and contractions of the urethra, the passage through the penis through which both urine and semen pass out of the male body.
The force and amount of ejaculate vary widely from male to male. A normal ejaculate may contain anywhere from 5 to 15 milliliters (from a teaspoon to a tablespoon). However, some men may barely generate a few small drops, while some men may ejaculate much more than this. The amount of ejaculate is highly dependent on how recent the last previous ejaculation was within a range of the first two days. In other words, an ejaculation soon after a previous ejaculation will be very small, but will be much larger if ejaculation has not occurred for two days or longer.
Some men may be able to shoot their ejaculate a distance of several feet, while the semen may simply ooze out of some men's penises.
The number of sperm in an ejaculation also varies widely, depending on many factors, including the recentness of last ejaculation, the average warmth of the testicles, the degree and length of time of sexual excitement prior to ejaculation, the age, testosterone level, and general fertility of the subject, and the total volume of seminal fluid.
Most men experience a lag time of some half-hour or so between the ability to ejaculate consecutively. This is called a refractory period, during which it is difficult or impossible to attain an erection.
There are wide variations in how long sexual intercourse can last before ejaculation occurs. Studies have shown that most men can only avoid ejaculation during active thrusting for five minutes or less. A minority can ejaculate more or less at will, and delay ejaculation for an hour or longer during sexual intercourse.
When man ejaculates before he wants to it is often called premature ejaculation. If a man is unable to ejaculate in a timely manner after prolonged sexual stimulation, in spite of his desire to do so, it is called delayed ejaculation.