Axis

[1] The centerline, whether real or imaginary, around which a thing rotates.
[2] One of the three axes of an aircraft, which are the straight lines through the center of gravity about which change of attitude occurs: longitudinal or drag axis in the plane of symmetry (roll); normal or lift axis vertically in the plane of symmetry (yaw); and the lateral or pitch axis transversely (pitch).
See also wind axes
[3] Of a lens, the line of symmetry of the optical system; the line along which there is no refraction.
See also cam-ground piston, eccentric bolt, hotchkiss drive, kingpin axis, kingpin inclination, moment of inertia, pivot axis, roll axis, self-aligning torque, static balance, steering-swivel axis, steering axis inclination, steering axis, steering geometry, steering knuckle angle., swivel axis, torque, trailing arm