1. n. 
A large valve at the top of a well that may be closed if the drilling crew loses control of formation fluids. By closing this valve (usually operated remotely via hydraulic actuators), the drilling crew usually regains control of the reservoir, and procedures can then be initiated to increase the mud density until it is possible to open the BOP and retain pressure control of the formation.
BOPs come in a variety of styles, sizes, and pressure ratings.
Because BOPs are critically important to the safety of the crew, the rig, and the wellbore itself, BOPs are inspected, tested, and refurbished at regular intervals determined by a combination of risk assessment, local practice, well type, and legal requirements. BOP tests vary from daily function testing on critical wells to monthly or less frequent testing on wells thought to have low probability of well control problems.
Alternate Form: BOP
See: annular BOP, blowout, BOP stack, casing string, drilling break, hook load, inside blowout preventer, kill line, mud density, nipple down, pipe ram, ram blowout preventer, shear ram, surface casing, underground blowout, wildcat