The Lagrangian, L, of a dynamical system is a function that summarizes the dynamics of the system. It is named after Joseph Louis Lagrange. The concept of a Lagrangian was introduced in a reformulation of classical mechanics introduced by Joseph Louis Lagrange in 1788, known as Lagrangian mechanics.
Read more about Lagrangian: Definition, Explanation, Lagrangian of A Test Particle, Lagrangians and Lagrangian Densities in Field Theory, Selected Fields, Mathematical Formalism