Source field

In theoretical physics, a source field is a field $J$ whose multiple

$S_{source} = J\Phi$

appears in the action, multiplied by the original field $\Phi$. Consequently, the source field appears on the right-hand side of the equations of motion (usually second-order partial differential equations) for $\Phi$. When the field $\Phi$ is the electromagnetic potential or the metric tensor, the source field is the electric current or the stress-energy tensor, respectively.

Also, all Green's functions (correlators) may be formally found via Taylor expansion of the partition sum considered as a function of the source fields.