A field theory that is an approximation to some more fundamental field theory or string theory; the appropriate degrees of freedom to describe the physics at some length-scale (or, equivalently, energy-scale) are used, with the degrees of freedom at shorter length-scales (or, equivalently, higher energy-scales) being ignored. The Lagrangian for an effective field theory is called an effective Lagrangian. In principle, an effective field theory can be derived from a more fundamental theory using the renormalization group. There are many examples of effective field theories in quantum field theory and solid-state physics.

An example of an effective field theory is the theory of weak interactions (see fundamental interactions) proposed by Enrico Fermi in the 1930s, with this theory being applicable at low energies. In this example, the more fundamental underlying theory is the Weinberg–Salam model.