Plain English Meaning
It is the invisible line that goes through the middle of the propeller from front to back. The propeller turns around this line like a wheel turns around its axle.
Full Definition
The imaginary line running straight through the centre of the propeller hub, parallel to the ground when the aircraft is in level flight. The propeller blades extend outward from this axis, and the propeller spins around it.
Context Anchor
Seen in explanations of propeller motion, airplane control, and forces caused by a spinning propeller.
Derivation
‘Horizontal’ comes from the Latin horizon, meaning the boundary line where sky meets ground — so horizontal means level with the ground. ‘Axis’ comes from the Latin axis, meaning axle or . Together: the level line a thing pivots around.