In quantum electrodynamics, the vertex function is the one particle irreducible correlation function involving ψ, <math>\bar{\psi}<math> and the vector potential A.

<math>\Gamma^\mu\equiv -{1\over e}{\delta^3 \Gamma_{eff}\over \delta \bar{\psi} \delta \psi \delta A_\mu}<math>

It's unfortunate that the effective action Γ_eff and the vertex function Γ^μ happen to be described by the same letter.

The dominant (and classical) contribution to Γ^μ is the gamma matrix γ^μ, which explains the choice of the letter.