Xenoliths are fragments of older rocks included in younger igneous rocks. Xenoliths form when magma migrates within the lithosphere incorporating pieces of other rocks within it. Xenoliths can be, for example, rocks encountered during magma ascent or fragments of the wall rocks of a magma chamber. The term derives from the Greek xenos, ‘alien’, and lithos, ‘stone’. The related term, xenocrystal (or xenocryst), indicates an ‘alien crystal’, a crystal originated outside of the magma that was incorporated in it.

Xenoliths and xenocrysts may form in multiple ways. They may be fragments of refractory material survived to the process of melting in the source area of the magma (restite) or fragments of unrelated rocks encountered during magma ascent and emplacement. Xenoliths may also consist of cognate material, fragments of earlier igneous rocks like cumulates formed in the same magmatic system and then reworked by a younger magma pulse. In this latter case, it might be hard to distinguish xenoliths and glomerocrysts, aggregates of igneous crystals.