Normal mode signal is any type (other than common mode) that appears between a pair of wires, or on a single wire referenced to (or returned via) the earth, chassis, or shield. Normal-mode signals are read between two wires in a balanced or unbalanced transmission path. For a balanced 2-wire path, one wire is driven positive while the other is driven negative an equal amount, both with respect to a static or no-signal condition in which both lines assume the same voltage level relative to circuit common.
Differential mode signal appears differentially on a pair of wires in an ungrounded cable configuration. In some circles, differential mode noise has been given the title of "Normal Mode Noise".