Semen is an organic fluid, also known as seminal fluid, that usually contains spermatozoa. It is secreted by the gonads (sexual glands) and other sexual organs of male and can fertilize female ova. In humans, seminal fluid is contains several components besides spermatozoa; proteolytic and other enzymes as well as fructose are elements of seminal fluid which promote the survival of spermatozoa and provide a medium through which they can move or "swim." The process that results in the discharge of semen is called ejaculation.

The most common way to collect a semen sample is through masturbation, directing the sample into a clean cup.

A sample may also be collected during intercourse in a special type of condom known as a collection condom. Collection condoms are made from silicone or polyurethane, as latex is somewhat harmful to sperm. A third option for collecting a sample is through coitus interruptus (withdrawal). With this technique, the man removes his penis from his partner near the end of intercourse and ejaculates into a cup.

Sperm count, or sperm concentration, measures the concentration of sperm in a man's ejaculate. Anything over 20 million sperm per milliliter is considered normal. Motility, morphology, volume, pH and other parameters are also evaluated according to World Health Organization approved methods.

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