Pitch angle (or blade pitch) is the angle that a blade is twisted by, relative to the helicopter, in order to change the amount of lift that it generates. The pitch angle of a blade is changed primarily by inputs coming from the swashplate that cause the blade to feather (though it can also be changed dynamically by the flybar to stabilize the helicopter)
A blade's pitch angle is generally different from its angle of attack; while pitch angle is the amount the blade is feathered by (relative to the mast), the angle of attack is the pitch of the blade in relation to the relative wind of the blade's dynamic environment. For example, with 10 degrees collective pitch applied, but the main rotor tilted forwards 10 degrees to the horizontal, the angle of attack of the advancing blade is zero degrees, and the angle of attack of the retreating blade will vary depending on how fast the helicopter is travelling forwards or backwards.
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