The common-mode rejection ratio (CMRR) of a differential input indicates the capability of the input to reject input signals common to both input leads. A high CMRR is important when the signal of interest is a small voltage fluctuation superimposed on a (large) voltage offset. It is also important when relevant information is contained in the voltage difference between two signals, like audio transmission over balanced lines or serial communication like USB and CAN bus.
The CMRR is defined as the ratio of the differential gain Ad over the common-mode gain Acm:
The CMRR is given in decibels (dB) and the higher the CMRR value is, the better.