An optical system that neither converges nor diverges a parallel beam of light. Its effective focal length is infinite. Beam expanders are the most common afocal systems in a laser lab, taking in a small collimated beam and outputting a larger collimated beam without bringing the light to a focal point.
Heard on the job
The Galilean telescope design gives us an afocal beam expander without an internal focal point where air breakdown could occur.