What is an Amperian loop?

(N/A) An Amperian loop is an imaginary closed loop or path in space,chosen to apply Ampere's circuital law to calculate the magnetic field produced by a current distribution.
It is analogous to a Gaussian surface in electrostatics.
The loop is typically chosen such that the magnetic field $B$ is either constant along the path or perpendicular to it,simplifying the line integral $\oint B \cdot dl = \mu_0 I_{\text{enclosed}}$.