Definition 1 of 2
Plain English Meaning
Pressure pushing back against something that is trying to flow forward. In an engine, it's the resistance the exhaust gases meet as they try to leave the cylinders.
Full Definition
Pressure that builds up against the normal direction of flow in a system, opposing or restricting that flow. In aircraft engines, back pressure refers specifically to the resistance to exhaust gases leaving the cylinders through the exhaust system, which reduces engine efficiency when excessive.
Context Anchor
Used during takeoff, landing, climbing, and any maneuver where the pilot needs to raise or hold the nose.
Derivation
From 'back' (in the reverse direction) plus 'pressure'. The name describes exactly what it does — pressure acting backward against the flow.