nonperturbative

This term is used exclusively within the highly specialized register of theoretical physics and mathematics. It describes a regime where the interaction between particles or fields is so strong that the standard method of starting with a simple system and adding small corrections—known as perturbation theory—fails completely.

In a practical sense, a nonperturbative effect is one that cannot be captured by any finite number of terms in a power series. It often refers to phenomena like solitons or instantons, which represent fundamental shifts in the state of a system that are invisible to perturbative calculations.