Definition
The three imaginary lines passing through an airplane's center of gravity around which it rotates: the longitudinal axis (nose to tail), the lateral axis (wingtip to wingtip), and the vertical axis (top to bottom). Rotation about the longitudinal axis is roll, about the lateral axis is pitch, and about the vertical axis is yaw.
Plain English
Every airplane can move in three ways in the air: it can tilt its wings up or down, raise or lower its nose, or swing its nose left or right. The 'three axes' are the invisible lines those movements happen around, all crossing at the airplane's balance point.
Context Anchor
You encounter this when learning how the flight controls affect the airplane during takeoff, climb, turns, and landing.
Derivation
Axis comes from the Latin 'axis,' meaning the central line a wheel turns around. The same idea applies here: each of the three axes is a line the airplane rotates around in flight.
Why Pilots Care
Control of all three axes prevents unwanted motion that can cause loss of directional control or runway departure during takeoff.
Intuition Check
The three axes are not three airplane parts or three directions of travel. They are three imaginary lines the airplane rotates around.
Example Sentence 1
An airplane in flight can rotate around any of the three axes, and the pilot uses ailerons, elevator, and rudder to control that rotation.
Example Sentence 2
Flight training teaches students how each primary control surface moves the airplane around one of the three axes.