Refer to figure.
Longitudinal or roll axis: A straight line through the centre of gravity of the helicopter from the nose to the tail about which the helicopter can roll left or right. Sometimes known as the X axis.

Lateral, transverse or pitch axis: Straight line through the centre of gravity of the helicopter about which the helicopter can pitch its nose up or down. This axis is also perpendicular to the reference plane of the helicopter, sometimes known as the Y axis.

Normal or Yaw axis: straight line through the centre of gravity, perpendicular to the plane defined by the longitudinal and lateral axes and about which the helicopter can yaw. Sometimes known as the Z axis.