Plain English Meaning
How much fuel is being mixed with the air going into the engine. The pilot can adjust this with a knob or lever in the cockpit to add more fuel, less fuel, or shut the fuel off completely.
Full Definition
The ratio of fuel to air being delivered to the engine's cylinders for combustion, controlled by the pilot via the mixture control in the cockpit. The mixture can be made richer (more fuel relative to air) or leaner (less fuel relative to air), and pulling the control fully out cuts off fuel entirely to shut down the engine.
Context Anchor
Seen in engine operation, after-landing, and , especially when the calls for moving the mixture to .