You have no doubt heard some of the following terms bandied about when people are talking about props. Here they are, correctly defined. This is the distance across the circle made by the blade tips as the propeller rotates. Another way to measure diameter would be to measure from the center of the propeller to the tip of one of the blades and multiply by two. This is the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw through wood.
Constant pitch means the pitch is the same at all points from the leading edge to the trailing edge.
Progressive pitch starts low at the leading edge and progressively increases to the trailing edge. The pitch number assigned is the sum of the average pitch over the entire blade. Nicks in the leading edge, bend or deformed blades, too much cup, sharp leading edge corners or incorrect matching of propeller style to the boat and motor, can cause cavitation. The objective when selecting a prop is to determine what propeller size will maximize your boat's performance while allowing your engine to operate in the factory recommended rpm range. Try this little test with your boat fueled and loaded with all your usual gear.