Understanding Common-"mode"
"Common-mode" has absolutely nothing to do with mean, median, mode, or statistics in general.  Instead, it means that a common signal is present in a differential circuit.  And "differential" has nothing to do with Calculus, it just means a circuit where we are interested in the difference between two signals.

Example:  two inputs, one at 5V, and one at 6V.  We could say that these have a common-mode signal of 5.5V, and a differential signal of 1V.  If the inputs are at -3V and -1V, the common-mode signal could be -2V and the differential signal is still 1V.  

Common-mode signals are often seen as interfering signals on a long twisted-pair cable.  The interference couples equally to both wires.  We use a differential input circuit to reject the common-mode interference.