Plain English Meaning
A propeller is shaped like a screw. If it were turning through something solid, one full turn would move it forward a fixed distance. But air isn't solid, so each turn actually moves the airplane forward a bit less than that. Slippage is that shortfall.
Full Definition
The difference between the geometric pitch of a propeller (the distance it would advance in one revolution if it were moving through a solid medium) and its effective pitch (the distance it actually advances through the air in one revolution). Slippage represents the loss of efficiency caused by the propeller working in a fluid rather than a solid.
Context Anchor
Seen in basic propeller discussions when comparing the propeller’s intended forward movement with the airplane’s actual forward movement.
Derivation
From the everyday word 'slip' — to slide without gripping. The propeller blade '' through the air rather than biting into it the way a screw bites into wood. Slippage is the amount of that .