Definition
Adjusted to a reduced fuel-to-air ratio by moving the mixture control toward the lean position, decreasing the amount of fuel delivered to the engine relative to the air entering it.
Plain English
The pilot has reduced the amount of fuel mixed with the air going into the engine, so the engine is running on a thinner fuel mixture.
Context Anchor
Seen in after-landing and taxi procedures, especially after the airplane is clear of the runway and stopped.
Derivation
From the everyday sense of 'lean' meaning thin or with less fat. A leaned mixture has less fuel in proportion to air -- a 'thinner' mixture.
Why Pilots Care
Leaning prevents spark-plug fouling, reduces fuel consumption, and avoids engine roughness or damage from an overly rich mixture while taxiing.
Intuition Check
Leaned does not mean the airplane is tilted or the pilot is leaning their body. Here it means the engine is being given less fuel in its air-fuel mix.
Example Sentence 1
After clearing the runway and stopping, the pilot leaned the mixture to prevent spark plug fouling during taxi.
Example Sentence 2
With the mixture leaned, the engine idled smoothly and used far less fuel during ground operations.