A monolith is a geological feature consisting of a single massive stone or rock, such as some mountains, or a single large piece of rock placed as, or within, a monument or building. Erosion usually exposes the geological formations, which are most often made of very hard and solid metamorphic or igneous rock.

In architecture, the term has considerable overlap with megalith, which is normally used for prehistory, and may be used in the contexts of rock-cut architecture that remains attached to solid rock, as in monolithic church, or for exceptionally large stones such as obelisks, statues, monolithic columns or large architraves, that may have been moved a considerable distance after quarrying. It may also be used of large glacial erratics moved by natural forces.

The word derives, via the Latin monolithus, from the Ancient Greek word μονόλιθος (monolithos), from μόνος ("one" or "single") and λίθος ("stone").