It is another word for sweat.

cooling mechanism of the body, also known as sweating or transpiration

salty fluid secreted by sweat glands; "sweat poured off his brow"

the process of the sweat glands of the skin secreting a salty fluid; "perspiration is a homeostatic process"

Unlike humans, birds do not regulate their body temperature with dermal sweat glands that secrete perspiration. Instead, they thermoregulate by giving off excess heat in the form of water vapor, mainly by panting. On really hot days, martins sit with their beaks agape (they are panting to facilitate water vapor loss from their mouth linings), in an effort to cool off. When martin nestlings get too hot (especially those being reared in uninsulated housing), they will spread their mouths open as extremely as a fully-opened book.

1) The secretion of fluid by the sweat (sudoriferous) glands. These small, tubular glands are situated within the skin, as well as in the subcutaneous tissue under it. They discharge their fluid through tiny openings in the surface of the skin. Perspiration serves at least two purposes: the removal of waste products such as urea and ammonia, and cooling of the body temperature as sweat evaporates. 2) The transparent, colorless, acidic fluid secreted by the sweat glands. It contains some fatty acids and mineral matter. Adult perspiration gains its characteristic odor from the waste products excreted. Also known as sweat.

Sweat, the salty fluid excreted from the sweat glands of the skin.

A mixture of water, salt, and waste products produced by sweat glands in the skin.