Manually operated facet or valve to drain water or air pressure.

A small, manually-operated faucet or valve for draining off liquids or releasing air pressure. The air-relief valve on a filter is an example.

The fuel valve, usually found on the side of the gas tank.

A petcock is a small valve used to control the flow of liquid or gas. Although petcocks are used in a wide variety of applications, the following passage will describe one of the most common applications of the petcock which is the control of gasoline on a motorcycle engine.