A voltage or current that applies identically to all the conductors (including return conductors and shields) associated with a cable, or with an item of equipment.
(Noise), caused by a difference in "group potential". By Ground at either end rather than both ends (usually grounded at source) one can reduce this interference.
The term refers to electrical interference which is measurable as a ground referenced signal. In true common mode a signal is common to both the current carrying conductors.