Cisalpine is the part of continental Europe situated North of Italy, embraced by the Alps.
It coincides with the Po Valley and historical Lombardy.
"Certum est tamen, Liguriam et partem Venetiae, Emiliam quoque Plamminiamque veteres historiographos Galliam Cisalpinam appellasse. [...]Sicut enim dicimus Galliam Transalpinam, quae ultra Alpes habetur, sic Galliam Cisalpinam hac parte, quae infra Alpes est, vocitamus. "
"It is certain, moreover,that the old writers of history called Liguria and part of Venetia, as well as Emilia and Flamminia, Cisalpine Gaul[...]For as we call what is beyond the Alps, Transalpine Gaul, so we name what is within the Alps on this side, Cisalpine Gaul."
Paul the Deacon, Historia Langobardorum, Book II chapter XXIII