Plain English Meaning
EPR is a number that compares the air pressure leaving the engine to the air pressure entering it. The bigger the difference, the more thrust the engine is producing.
Full Definition
In a turbojet or turbofan engine, the ratio of turbine discharge (exhaust) total pressure to compressor inlet total pressure. EPR is used as a primary indication of thrust output on many jet engines and is displayed on a dedicated cockpit gauge.
Context Anchor
Seen on turbine-engine cockpit instruments, power-setting charts, and procedures that tell the pilot what to set.