A xenolith is a rock fragment which becomes enveloped in a larger rock during the larger rocks development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption. To be considered a true xenolith, the included rock must be identifiably different from the rock in which it is enveloped.
Above, the xenolith is biotite schist, enveloped in the Almo Pluton. (photo NPS.gov)