Collimated light

Collimated light is light whose rays are parallel, and therefore will spread minimally as it propagates. The word is related to "collinear" and implies light that does not disperse with distance (ideally), or that will disperse minimally (in reality). A perfectly collimated beam with no divergence cannot be created due to diffraction, but light can be approximately collimated by a number of processes, for instance by means of a collimator. Collimated light is sometimes said to be focused at infinity. Thus as the distance from a point source increases, the spherical wavefronts become flatter and closer to plane waves, which are perfectly collimated.
The word "collimate" comes from the Latin verb collimare, which originated in a misreading of collineare, "to direct in a straight line"

Laser light from gas or crystal lasers is highly collimated because it is formed in an optical cavity between two parallel mirrors, in addition to being coherent. In practice, gas lasers use slightly concave mirrors, otherwise the power output would be unstable due to mirror non-parallelism from thermal and mechanical stresses. The divergence of high-quality laser beams is commonly less than 1 milliradian, and can be much less for large-diameter beams. Laser diodes emit less-collimated light due to their short cavity, and therefore higher collimation requires a collimating lens.