Plain English Meaning
A propeller blade is one of the spinning arms of the propeller. It is shaped like a small wing and is twisted from root to tip so that, as it turns, it pulls the airplane forward through the air.
Full Definition
One of the airfoil-shaped arms that extend outward from the propeller hub. Each blade has a leading edge, a trailing edge, a tip, and a root (where it joins the hub), and is twisted along its length so that every section meets the oncoming air at an effective angle as the propeller rotates. As the engine spins the propeller, each blade acts like a small rotating wing, producing thrust by accelerating air rearward.