1. In the mathematical sense, an axis is a line with respect to which a curve or figure is drawn, measured, rotated, etc. The word comes from the Greek root aks for a point of turning or rotation and seems to have first been used in English by Thomas Digges around 1570 in reference to the rotational axis of a right circular cone. In plane analytical geometry (and graphs) two axes, usually at right angles, are most commonly used. In three-dimensional geometry, three axes, usually at mutual right angles, are most common. Their point of intersection is the origin.
2. In astronomy and geography, an axis is an imaginary straight line through the center of celestial body, such as a planet or star, around which the body rotates. The axial period is the time taken for an object to make one complete rotation on its axis. The axial period of a planet is its "day". In mechanics, an axis runs longitudinally through the center of an axle or rotating shaft.
3. In terms of flight, an axis is one of three lines of reference around which an aircraft or spacecraft can rotate. These lines are the longitudinal (front-to-back), horizontal, and vertical axes. Rotation along them is known as roll, pitch, and yaw, respectively.