Plain English Meaning
How well a fuel resists exploding on its own inside the engine before the spark plug fires. The higher the antiknock value, the better the fuel handles heat and pressure without causing knocking.
Full Definition
A measure of an aviation fuel's resistance to detonation (uncontrolled, spontaneous combustion) inside the engine cylinders. Higher antiknock value means the fuel can withstand greater heat and pressure before igniting on its own, allowing the engine to run smoothly without damaging knocking.
Context Anchor
Seen when studying aviation gasoline grades and choosing the correct fuel for a piston-engine aircraft.
Derivation
From 'anti-' (against) and 'knock' -- the audible knocking or pinging sound an engine makes when fuel detonates at the wrong . The term simply names the property of fighting against that knocking.