Definition
An aircraft in flight is free to rotate in three dimensions:
Moving the nose to the left or the right, on a axis running up and down the aircraft, is Yaw (or the vertical axis).
Moving the nose up or down about an axis running from wing to wing is Pitch (or the transverse axis).
Rotating about the axis running from nose to tail results in Roll (or the longitudinal axis).
Specifics
Vertical Axis (Yaw):
The yaw axis has it origin at the center of gravity and is directed towards the bottom of the aircraft. It is perpendicular to the wings and to the fuselage reference line.
A positive yawing motion moves the nose of the aircraft to the right.
The rudder is the primary control of yaw.
Transverse Axis (Pitch):
The pitch axis has its origin at the center of gravity and is directed to the right, parallel to a line drawn from wingtip to wingtip.
A positive pitching motion raises the nose of the aircraft and lowers the tail.
The elevators are the primary control of pitch.
Longitudinal Axis (Roll):
The roll axis has its origin at the center of gravity and is directed forward, parallel to the fuselage reference line.
A positive rolling motion lifts the left wing and lower the right wing.
An angular displacement about this axis is called bank. The pilot can change the bank angle by increasing lift on one wing and decreasing it on the other.
The ailerons are the primary control of bank, however, the rudder can have a secondary effect on bank.
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