rays lying close to the axis of a centered optical system and forming very small angles with the axis and with the normals to the refracting and reflecting surfaces of the system.

The main relations describing the formation of optical images in centered, or axisymmetric, systems are strictly speaking valid only for paraxial rays in the limiting case of infinitesimal angles. In particular, only paraxial rays represent a point as a point, a straight line as a straight line, and a plane as a plane. Only in images produced by such rays are there no optical aberrations (apart from chromatic aberration in lens systems). In practice, however, paraxial rays are usually taken to be rays that form finite angles of up to a few degrees where the deviations from the strict relations are so small that they may be disregarded; the criteria for smallness are different for each specific case. The region about the optical axis of a system where the rays may be considered paraxial is called the paraxial, or Gauss, region.