a shaft on which a wheel rotates
"The mechanic tightened the bolts holding the axle, ensuring the wheel would rotate smoothly without wobbling."
Shoulder.
"The mechanic tightened the bolt that secures the axle to ensure the wheel spins smoothly without wobbling."
The pin or spindle on which a wheel revolves, or which revolves with a wheel.
In plain English: An axle is the metal bar that holds your wheels on and lets them spin inside a vehicle's frame.
"The mechanic tightened the loose axle on my truck before I could drive away."
Usage: This term refers specifically to the mechanical rod connecting wheels and should not be confused with similar-sounding words like "axis." It is primarily used in technical contexts regarding vehicles rather than general conversation.
The word "axle" comes from Old English words meaning "shoulder," which eventually shifted to describe the metal rod that connects a vehicle's wheels. This journey into modern usage reflects how mechanical parts were named after their round, shoulder-like shape rather than retaining any literal connection to human anatomy today.