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.