One of the three lines of reference around which an aircraft can rotate. The three movements an aircraft can make (other than moving straight ahead) all involve rotation around some central point. That point is called an axis. These movements are:
Roll: rotation around the longitudinal (front-to-back) axis (nose remains pointed forward, but the rocket spins),
Pitch: rotation around the horizontal axis (nose moves up or down), and
Yaw: rotation around the vertical axis (nose moves left or right.).