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Semen is the fluid discharged by the male reproductive tract during ejaculation. In humans it normally contains sperm suspended in secretions from reproductive glands, including the prostate and seminal vesicles. Semen is therefore a mixture, whereas a sperm is an individual gamete.^[ref]

Fluid volume and the number or movement of sperm describe different properties. A sample can contain fluid but few or no sperm, so visible semen alone does not establish successful gametogenesis or fertilization.

A seminal vesicle is a paired accessory gland that contributes fluid to semen. Its secretion includes fructose and other substances that help form the environment in which sperm are transported. The gland's duct joins the sperm-carrying pathway near the prostate.^[ref]

Despite its name, a seminal vesicle is not primarily a storage sac for sperm. Sperm production, maturation, movement through ducts and addition of accessory-gland fluid are distinct contributions to reproduction. This distinction also explains why semen volume and sperm count describe different quantities: much of the ejaculated fluid is glandular secretion rather than sperm cells themselves.