Definition

Axis, as applied to aviation, is defined as "an imaginary line about which a body rotates".

Discussion

An aircraft in flight manoeuvres in three dimensions. To control this movement, the pilot manipulates the flight controls to cause the aircraft to rotate about one or more of its three axes of rotation. These three axes, referred to as longitudinal, lateral and vertical, are each perpendicular to the others and intersect at the aircraft centre of gravity.

Axes of Rotation. Source: Wikicommons