1. n. 
A set of two or more BOPs used to ensure pressure control of a well. A typical stack might consist of one to six ram-type preventers and, optionally, one or two annular-type preventers. A typical stack configuration has the ram preventers on the bottom and the annular preventers at the top.
The configuration of the stack preventers is optimized to provide maximum pressure integrity, safety and flexibility in the event of a well control incident. For example, in a multiple ram configuration, one set of rams might be fitted to close on 5-in diameter drillpipe, another set configured for 4 1/2-in drillpipe, a third fitted with blind rams to close on the openhole, and a fourth fitted with a shear ram that can cut and hang-off the drillpipe as a last resort.
It is common to have an annular preventer or two on the top of the stack since annulars can be closed over a wide range of tubular sizes and the openhole, but are typically not rated for pressures as high as ram preventers. The BOP stack also includes various spools, adapters and piping outlets to permit the circulation of wellbore fluids under pressure in the event of a well control incident.
See: annular BOP, blind ram, blowout preventer, casinghead, cellar, drilling riser, dynamic positioning, kill line, nipple up, ram blowout preventer, ram preventer, shut-in bottomhole pressure, shut-in pressure, Texas deck