Faddeev-Popov determinant
= Faddeev-Popov determinant
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{title2=$\Delta_{\mathrm{FP}}$}
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The Faddeev-Popov determinant is the functional Jacobian
$$
\Delta_{\mathrm{FP}}[A]=\det\left(\frac{\delta G(A^\alpha)}{\delta\alpha}\right)_{G=0}
$$
that compensates for the change from integration along a gauge orbit to a gauge-fixing condition $G(A)=0$. It can be represented by a path integral over a <Faddeev-Popov ghost field> pair.