1. A stream of radiation or particles confined to a narrow range of directions.

2. An imaginary stream of radiation originating at the focus of a telescope (or central point of an array of telescopes) and illuminating the volume of space from which the telescope or array can receive radiation. The term is commonly used to refer to the patch of sky to which the telescope is sensitive. Optical and infrared telescopes generally have a simple circular beam, while radio telescopes (especially interferometers) may have beams of a complex shape. See also Fan Beam.