Define propeller slip

The distance a ship has traveled in the last 24h w / what it theoretically should've traveled based on its pitch

Prop slip is not the same as propeller efficiency. If the prop did not slip at all as it screws through the water, each rev theoretically propels the boat the forward a distance equal to the prop pitch. (a 15 inch pitch prop theoretically propels the boat 15 inches in one revolution)