Basalt ( /bəˈsɔːlt/, /ˈbæsɒlt/, /ˈbæsɔːlt/, or /ˈbeɪsɔːlt/) is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to the fast cooling of lava at the surface of a moon or planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey. By definition, basalt is defined as an aphanitic igneous rock that contains, by volume, less than 20% quartz and less than 10% feldspathoid and where at least 65% of the feldspar is in the form of plagioclase.

On Earth, most basalt magmas have formed by decompression melting of the mantle. Basalt commonly erupts on Io, and has also formed on Earth's Moon, Mars, Venus, and even on the asteroid Vesta. Source rocks for the partial melts probably include both peridotite and pyroxenite (e.g., Sobolev et al., 2007). The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below ocean ridges.

The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarse) groundmass are generally referred to as diabase (also called dolerite) or gabbro.