An extended source is an astronomical object presenting an angular size, covering an area of the celestial sphere rather than appearing as a point with no size. This can be determined if the telescope has sufficient angular resolution (so an astronomical object might be an extended source for one telescope but a point source for another) or if some extent to its size can be determined by repointing a telescope. The latter is a technique of radio astronomy, where the term is common, and the term may be taken to mean the source extends beyond the radio telescope's FOV (i.e., its beam). A point source is the converse. The Sun and Moon technically extended sources, but the terms are conventionally used for more distant objects, such as those beyond the solar system: clouds, stellar clusters, galaxies, galaxy clusters, etc. might be extended sources, and stars generally point sources. Within the solar system, the terms might be used in discussions of distant minor planets.
The terms resolved source and unresolved source mean the same as extended source (implying a finite angular size) and point source, respectively.