LISN (line impedance stabilization network) is a device used in conducted and radiated radio-frequency emission and susceptibility tests, as specified in various EMC/EMI test standards (e.g. by CISPR, IEC, CENELEC, FCC, MIL-STD, etc). 

A LISN is a low-pass filter typically placed between an AC or DC power source and EUT (Equipment Under Test) to create known impedance and to provide an RF noise measurement port. It also isolates the unwanted RF signals from the power source. 

In addition, LISNs can be used to predict conducted emission for diagnostic and pre-compliance testing.