Common-mode voltage is a term that seems to come from the world of differential amplifiers but finds itself being used generically in the EMC domain.
An ideal differential amplifier only sees the difference between its inputs:
$$v_o = A (v_h – v_l)$$
Each of the two inputs can be defined as two common components, a differential and a common mode.
$$v_h = v_{cm} + \frac{v_{id}}{2}$$
$$v_l = v_{cm} – \frac{v_{id}}{2}$$
The differential amplifier then ideally outputs:
$$v_o = Av_{id}$$
In reality, there is a finite common-mode rejection ratio, CMRR, such that:
$$v_o = Av_{id} + CMRRv_{cm}$$