Diameter
Diameter is two times the distance from the center of the hub to the tip of the blade. It can also be looked at as the distance across the circle that the propeller would make when rotating. It is the first number listed when describing a propeller.

Hub
Solid cylinder located at the center of the propeller. Bored to accommodate the engine shaft. Hub shapes include cylindrical, conical, radius, & barreled.

Leading Edge
The edge of the propeller blade adjacent to the forward end of the hub. When viewing the propeller from astern, this edge is furthest away. The leading edge leads into the flow when providing forward thrust.