1 A source of angular size greater than the resolution of the instrument used to observe it; therefore, an extended source is said to be resolved. To the human eye, the Sun and Moon are extended sources, but the stars and planets are not. See also Point Source.
2 In radio astronomy, a class of sources of large angular extent displaying extended structure such as jets and lobes, and tending to be brightest at long wavelengths. See also Compact Source.