Orifice

An orifice is any opening, mouth, hole or vent, as in a pipe, a plate, or a body

Body orifice, any opening in the body of a human or animal
Orifice plate, a restriction used to measure flow or to control pressure or flow, sometimes given specialised names:
Calibrated orifice, used to control pressure or flow
Restrictive flow orifice, used to control flow
Miss Shilling's orifice, used to control flow in the engines of early Spitfire and Hurricane fighter aeroplanes
Back Orifice, a tendentious computer program designed for remote system administration

See also