Plain English Meaning
The small amount of fuel that stays trapped in the tanks and lines and can never reach the engine, no matter how full you fill up. It still has weight, so it counts as part of the aircraft itself, not as part of your usable fuel load.
Full Definition
The fuel remaining in an aircraft's fuel system that cannot be safely delivered to the engine in flight. It is the portion of total fuel onboard that is not available for use because of tank shape, fuel pickup location, or aircraft attitude limitations. Unusable fuel is determined by the manufacturer during certification and is included in the aircraft's empty weight.
Context Anchor
You will see this term in aircraft specifications, weight-and-balance , , and .