Plain English Meaning
Water that is mixed into the fuel as fine droplets, floating along with it instead of pooling at the bottom where it could be drained off.
Full Definition
Water that is suspended in fuel as tiny droplets, dispersed throughout the fuel rather than settled at the bottom of the tank. The droplets are small enough to remain mixed in the fuel and are not removed by simple draining of sumps.
Context Anchor
Seen in powerplant and fuel-contamination discussions, especially when checking fuel samples, fuel drains, filters, and water separators.
Derivation
From the French entrainer, meaning to drag or carry along. The water is being carried along by the fuel — moving with it as a passenger rather than separating out.
Why Pilots Care
Undrained entrained water can reach fuel lines and carburetors, causing rough running, power loss, or in the engine.