An extended source is a source whose angular size exceeds the resolution of the instrument being used to observe it. Extended sources, such as many galaxies and nebulae, can be resolved by telescopes, whereas most stars cannot and are therefore classes as point sources.

Also, in radio astronomy, an extended source was formerly a source whose angular extent could be measured, as distinguished from a point source. Now, it is regraded as one that has a large angular extent and is strongest at longer wavelengths, as distinguished from a compact source.