Plain English Meaning

The pilot is intentionally pushing the rudder pedal one way and rolling the wings the other way, so the airplane flies a little sideways instead of straight ahead. This is the control input that creates a slip.

Full Definition

The deliberate use of rudder and aileron in opposing or unmatched amounts so that the airplane is no longer flying in coordinated flight. In a slip, for example, the pilot banks the wings in one direction with aileron while applying rudder in the opposite direction, causing the airplane to fly partially sideways through the air rather than straight along its longitudinal axis.

Context Anchor

Seen in discussions of intentional slips, crosswind landings, and coordinated flight control use.