A collimated light beam is a beam where the rays stay parallel. This minimizes the spread of a collimated light beam as it propagates. A perfectly collimated light beam, with no divergence, wouldn't spread at all over distance. Unfortunately, diffraction prevents this from happening.
Light can be approximately collimated by a number of processes, for instance by means of a collimator. Perfectly collimated light is sometimes said to be focused on infinity. Thus, as the distance from a point source increases, the spherical wavefronts become flatter and closer to plane waves, which are perfectly collimated.
Other forms of electromagnetic radiation can also be collimated. In radiology, X-rays are collimated to reduce the volume of the patient's tissue that is irradiated and to remove stray photons that reduce the quality of the x-ray image ("film fog"). In scintigraphy, a gamma-ray collimator is used in front of a detector to allow only photons perpendicular to the surface to be detected.
The word "collimate" comes from the Latin verb collimare, which originated in a misreading of collineare, "to direct in a straight line".
A perfect parabolic mirror will bring parallel rays to a focus at a single point. Conversely, a point source at the focus of a parabolic mirror will produce a beam of collimated light creating a Collimator. Since the source needs to be small, such an optical system cannot produce much optical power. Spherical mirrors are easier to make than parabolic mirrors and they are often used to produce approximately collimated light. Many types of lenses can also produce collimated light from point-like sources.