The principle of least action is a fundamental concept in physics and mathematics that states that the path taken by a system between two states is the one for which the action is minimized. Action is defined as the integral of the Lagrangian, which is a function that summarizes the dynamics of the system. This principle helps to derive the equations of motion for various physical systems. In essence, the principle suggests that nature operates in the most efficient way possible. It applies to a wide range of fields, including classical mechanics, quantum mechanics, and even economics, influencing how we understand the behavior of systems from particles to planets.