Electronic #noise is anything that's not the #signal - it's a voltage or current excursion from the norm. In terms of power, it's ripple and variation from either the #DC voltage or the #AC sineusoid.

common mode noise

Common mode noise is present on both lines in the same direction.

A common mode choke impedes noise that impacts BOTH lines while allowing currents that are in opposite directions.

differential mode noise

differential mode noise is present on both lines with opposite direction.