An apophysis is a small, offshoot intrusion of igneous rock that extends from a larger igneous body, such as a batholith or pluton. Apophyses are significant in studying the intrusion and cooling processes of igneous rocks, as well as in understanding the complex geometry of magmatic systems.
Reference: Best, M. G. (2003). Igneous and Metamorphic Petrology. Wiley-Blackwell.