A collimator is a device that narrows a beam of particles or waves. To "narrow" can mean either to cause the directions of motion to become more aligned in a specific direction (i.e. collimated or parallel) or to cause the spatial cross section of the beam to become smaller.
Autocollimation is a process by which the focal length of a diverging lens is determined. Autocollimation involves placing a light source at the focal length of a converging lens on one side and a screen at the same distance on the other side so that the image produced is the sharpest possible image. When this is achieved, the screen is replaced with a mirror and the diverging lens is inserted between the converging lens and the light source so that the light rays that hit the mirror are parallel to the principal axis and when the light returns through the diverging and converging lenses, it produces a sharp image on top of the luminous object.