Blade pitch or simply pitch refers to turning the angle of attack of the blades of a propeller into or out of the wind to control the production or absorption of power. Wind turbines use this to adjust the rotation speed and the generated power. A propeller of a ship uses this effect to control the ship's speed without changing the rotation of the shaft and to increase the efficiency of streaming fluids.

In aircraft, blade pitch is usually described as "coarse" for a high angle of attack, and "fine" for a low angle of attack.

Blade pitch is normally described in units of distance/rotation assuming no slip.

Blade pitch acts much like the gearing of the final drive of a car. Low pitch yields good low speed acceleration (or climb rate in an aircraft) while high pitch optimizes high speed performance and economy.

Because the velocity of a propeller blade varies from the hub to the tip, they must be of twisted form in order for the pitch to remain constant along the length of the blade. This is typical of all but the crudest propellers.

It is quite common in aircraft for the propeller to be designed to vary pitch in flight, optimizing both cruise and takeoff performance.

Some Composite propeller have interchangeable blades, which enables the blade pitch to be adjusted. Changing the blade pitch in different elevations can be very beneficial to a boater. Typically, using a lower pitch will provide better hole shot. In contrast, a higher pitch enables better top end speed for the motor.In higher elevations, a lower pitch propeller will provide better performance.

The angle of attack is often confused with the pitch angle of an aircraft. Pitch angle is measured from the same reference line as angle of attack (conceptually the chord line), but to the horizon. Whereas, the angle of attack is measured from the (same) reference line to the direction of the arrival of the air to the wing (caused by the relative motion between the wing and the atmosphere).

In straight and level flight (through air that is not rising or falling), pitch angle and angle of attack have the same values. In this situation, the air (the relative wind) happens to be arriving toward the aircraft from the direction of the horizon, and so the two angles are the same in size. In straight descending flight, angle of attack becomes greater than pitch angle because the air arrives at the aircraft from below the horizon. Conversely, in straight ascending flight, angle of attack becomes less than pitch angle.

Diagram of Angle of attack

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)

For More Information: Angle of Attack & Lift Coefficient