Plain English Meaning

A spinning propeller behaves like a gyroscope. If you try to tilt it one way, the aircraft reacts as if you had pushed it ninety degrees later in the direction the prop is spinning. So a pitch input can show up as a yaw, and a yaw input can show up as a pitch.

Full Definition

The tendency of a spinning mass, such as a propeller, to resist a change in the direction of its axis and, when a force is applied to tilt that axis, to react with a force felt 90 degrees ahead in the direction of rotation. In a propeller-driven aircraft, this means a yawing or pitching input applied to the propeller disc produces a reaction in a different plane than the one in which the force was applied.

Context Anchor

Seen in discussions of propeller airplane handling, especially takeoff, operations, and rapid or changes.