The fluid produced by the male which carries the sperm to the female, often resulting in pregnancy.
Sperm are produced in the testicles, and most of the fluid in the prostate. At the time of male orgasm, semen is ejected from the penis (this is called "ejaculation").
All sexual behaviors that result in the contact of semen with the vagina or vulva may result in pregnancy at any time unless adequate contraceptive (birth control) measures are in force. Sexual intercourse is generally a very effective cause of pregnancy and should be avoided unless pregnancy is desired (see abstinence), unless other adequate contraceptive measures are in force.
When a man is sexually excited, a small amount of a clear fluid (pre-ejaculate) may leak out of the penis before orgasm and ejaculation. One reason coitus interruptus ("withdrawal" of the penis from the vagina just before the man's orgasm) is not a very effective contraceptive (birth control) method is because this pre-ejaculate fluid may also contain sperm.
Semen is a whitish fluid containing water and small amounts of salt, protein, and fructose sugar, and is in itself harmless on the skin or when swallowed. (It does sting if gotten into the eyes.)
However, semen can transmit sexually transmitted diseases, most notably AIDS. Any kind of sexual or other contact with the semen of a person infected with the HIV (AIDS) virus should be avoided.