Plain English Meaning
Anything in the fuel that shouldn't be there. Water, dirt, rust, or the wrong kind of fuel can all stop the engine from running properly.
Full Definition
The presence of any unwanted substance in aircraft fuel, most commonly water, dirt, rust, microbial growth, or fuel of the wrong type or grade. Even small amounts can disrupt engine operation by interfering with combustion or blocking fuel system components.
Context Anchor
Pilots look for fuel contamination during preflight fuel checks, after refueling, and any time the aircraft has been sitting where water, dirt, or other unwanted material could get into the tanks.
Derivation
From Latin contaminare, meaning 'to make impure by mixing.' Here it means the fuel has been mixed with something that doesn't belong, even in tiny amounts.
Why Pilots Care
Contaminated fuel can cause or loss at any .