The "Action Principle" is a concept in physics that states the path taken by a system between two states is the one for which the action is minimized or made stationary. Action is defined as the integral of the Lagrangian, which is a function that summarizes the dynamics of the system. This principle is foundational in classical mechanics and is used to derive the equations of motion for various physical systems. In the context of Lagrangian mechanics, the Action Principle provides a powerful method for analyzing motion without directly solving forces. It has also influenced other areas of physics, including quantum mechanics and general relativity, highlighting its broad applicability in understanding the behavior of physical systems.