In
physics, a
coupling constant or
gauge coupling parameter is a number that determines the strength of the
force exerted in an
interaction. Usually, the
Lagrangian or the
Hamiltonian of a system describing an interaction can be separated into a
kinetic part and an
interaction part. The coupling constant determines the strength of the interaction part with respect to the kinetic part, or between two sectors of the interaction part. For example, the
electric charge of a particle is a coupling constant.