Metamorphic grade refers to the degree of metamorphism that a rock has undergone, typically indicated by the presence of specific mineral assemblages that form under particular temperature and pressure conditions. The concept of metamorphic grade is essential for understanding the thermal history and tectonic setting of metamorphic rocks.
Reference: Bucher, K., & Grapes, R. (2011). “Petrogenesis of Metamorphic Rocks.” Springer.