Answer:
A fault causing a difference between the primary and secondary currents of a power transformer. Usually a leakage to earth from somewhere in a winding. Such errors are caught by a protective circuit comprising two current transformers for transformation of the primary and secondary currents respectively, in a common scale typically 0...1 or 0...5 A and bringing them together in a loop. A transverse branch in the middle of the loop collects the differential current set up in case of a fault, the so called 'spill' current. The differential branch uses this current to operate the operating winding of the protection relay and through this the alarm and trip contacts. The protection circuit thus simulates what is happening in the power circuit in an under graded and more direct issue.