Semen is the fluid excreted through the male penis, which, during vaginal intercourse, carries sperm into the female, sometimes resulting in pregnancy. 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. Indeed, it is widely believed to have beneficial qualities when applied to the skin. However, semen can be the vehicle for many sexually transmitted diseases, including HIV, the virus that causes AIDS. Any kind of sexual or other skin contact with the semen of a person infected with HIV should be avoided, even by persons already infected with the virus, as this may cause harmful re-infection.

Spermotozoa (or sperm, for short) are produced in the testicles, and most of the remaining fluid is produced by the prostate. At the time of male orgasm, semen is ejected through the urethra of 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. "Unprotected" sexual intercourse (that is, sex without the use of contraceptives) between a biological male and female is generally the most effective cause of pregnancy, unless contraceptive measures are in force. When a male is sexually excited, a small amount of a clear fluid (pre-ejaculate) may leak out of the penis before orgasm and ejaculation. One reason that coitus interruptus ("withdrawal" of the penis from the vagina just before the ejaculation of the penis) is not a very effective contraceptive method during heterosexual intercourse is that this pre-ejaculate fluid may also contain sperm.

In some cultures, semen is attributed with special properties of masculinity. For instance, some cultures in the South Seas have termed it jerungdu and believed that ingestion by boys was necessary for their proper sexual maturation. However, as noted above, semen can transmit sexually transmitted diseases.