Collimation

Collimation as related to astronomy is the alignment of the optical elements in a telescope, monocular or binocular such that light follows a pre-determined path accurately. The idea here is that some telescopes (such as reflector-style telescopes) have adjustable optical elements which may not always be properly aligned due to temperature changes, movement or age/use. When the telescope is "collimated," all of the optical elements (mirrors/lenses) focus the collected light to a single point for magnification through an eyepiece.

See the wikipedia article here