Device used to stop or control the rate of flow in a line or vessel or to serve as an automatic or semiautomatic safety device. Common valves include the butterfly, gate, plug, globe, needle, check, and pressure relief.
Source: IADC UBO / MPD Glossary, December 2011. Global Standards
A device for regulating or controlling the flow of material by means of a movable part or parts which open or close a passage.
Source: Petroleum Safety Orders–Drilling and Production, Definitions, California Code of Regulations, 8 CCR § 6505, December 2012. Regulations