Definition
The proportion, by weight, of air to fuel delivered to the cylinders of a reciprocating engine for combustion. It is expressed as a ratio such as 12:1, meaning twelve pounds of air for every one pound of fuel. The chemically ideal ratio for complete combustion of aviation gasoline is approximately 15:1, called the stoichiometric ratio. Ratios with more fuel than this are 'rich'; ratios with less fuel are 'lean'.
Plain English
How much air is being mixed with each unit of fuel before it burns inside the engine. More air relative to fuel is a lean mixture; more fuel relative to air is a rich mixture.
Context Anchor
Seen in piston engine operation, carburetor and fuel injection discussions, mixture control use, and engine troubleshooting.
Derivation
Ratio comes from a Latin word meaning a reckoning or comparison. That helps here because the term is not naming an amount of fuel by itself; it is comparing the amount of air to the amount of fuel.
Why Pilots Care
The correct ratio delivers full power, protects the engine from overheating or detonation, and controls fuel consumption.
Analogy
It is like a recipe: the result changes if you use too much or too little of one ingredient. The engine needs the air and fuel in the right proportion to burn properly.
Intuition Check
Do not read this as equal parts air and fuel. In normal engine operation, the mixture contains much more air than fuel, and the ratio is normally compared by weight, not by volume.
Example Sentence 1
As the aircraft climbed, the pilot leaned the mixture to maintain a proper air/fuel mixture ratio in the thinner air.
Example Sentence 2
A richer air/fuel mixture ratio is selected for takeoff to help cool the engine.