Plain English Meaning
When you push on a spinning object like a propeller, it doesn't react where you pushed it. The reaction shows up a quarter-turn later in the spin. That delayed, shifted reaction is the resultant action.
Full Definition
In the context of gyroscopic precession, the reaction of a spinning rotor (such as a propeller or rotor disc) to an applied force, where the effect appears not at the point the force was applied, but 90 degrees ahead in the direction of rotation. The 'resultant action' is the actual response that occurs after this 90-degree shift.
Context Anchor
Seen in discussions of gyroscopic precession, especially when explaining why a propeller-driven airplane may try to swing its nose left or right during pitch changes.