It seems like you're referring to the definition of the Lagrangian. In classical mechanics, the Lagrangian is a mathematical function that describes the dynamics of a system. It is defined as the difference between the kinetic energy (T) and potential energy (V) of a system:
L=T−VL = T - VL=T−V
This function is central to the Lagrangian mechanics formulation and is used to derive the equations of motion for a system through the Euler-Lagrange equation.
If you meant a different context for "Lagrangian," could you clarify so I can provide a more specific explanation?