Refer to figure.
The blades are free to move relative to the hub, the rotor is able to tilt left, right, forward and back. Therefore, we need to define more than one plane of rotation and associated axes.

Tip Path Plane: The tip path plane is the plane traced out by the blades tips. This plane is also known as the no-flapping plane or rotor disc plane. The tip path plane is parallel to the plane of rotation.

Rotor Disc: The rotor disc is traced out by the blade in the tip path plane.

Plane of Rotation: The plane of rotation, also known as the Hub Plane of Rotation is parallel to the tip path plane but runs through the centre of the hub. The Hub Plane (fixed reference between the hub and the shaft) is not the same as the hub plane of rotation.

Axis of Rotation: The axis of rotation is the axis about which the rotor rotates, it is perpendicular to the plane of rotation as well as the tip path plane and runs through the centre of the hub.

Blade Pitch Angle: The pitch angle is defined as the angle between the blade chord line and the plane of rotation which acts around the feathering axis.

Main Rotor Shaft: The main rotor shaft axis runs through the main rotor.