Dictionary.net

A valve in the boiler of a steam-engine: (1) external, held down by a regulated force so as to open when the steam pressure exceeds a certain amount, and thus both relieve the pressure and give notice to the engine-man; (2) internal, to relieve the pressure of the external air by letting in air when the cooling of the steam produces a vacuum within the boiler.

n. A valve fitted to the boiler, which opens and lets out the steam when the pressure within becomes too great for safety.