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.
Source Best, M. G. (2003). Igneous and Metamorphic Petrology. Wiley-Blackwell.