Abbreviation for Common Mode Rejection Ratio. In balanced lines there is always a positive signal on one cable and a negative or opposite polarity signal on the other. The idea is that anything common to both wires will eventually get canceled at the receiving end (as noted in WFTD Balanced). The degree of cancellation at the receiving end, however, is dependent upon a number of factors relating to the integrity of the circuit involved. CMRR is a spec that defines to what degree common signals are canceled at the input of a balanced system.