Axis, as applied to aviation, is defined as "an imaginary line about which a body rotates".
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
Motion around the longitudinal axis, the lateral axis and the vertical axis are referred to as roll, pitch and yaw respectively. The primary flight control surface for controlling roll is the ailerons, for pitch the elevator and for yaw, the rudder. Rotation around one axis may induce an uncommanded rotation around a second axis. In this case, simultaneous use of the associated flight control surfaces is required to maintain coordinated flight.