CMRR stands for common mode rejection ratio. It is a
measure of the ability of a test instrument to reject
interference that is common to both of its measurement
input terminals. It is expressed in decibels and it is the
ratio of the actual or common signal level appearing on the
two input terminals together to the measured level.

The common-mode rejection ratio (CMRR) of a differential amplifier (or other device) is the tendency of the devices to reject the input signals common to both input leads. A high CMRR is important in applications where the signal of interest is represented by a small voltage fluctuation superimposed on a (possibly large) voltage offset, or when relevant information is contained in the voltage difference between two signals.
The CMRR is defined as the ratio of the powers of the differential gain over the common-mode gain, measured in positive decibels.
The equation of CMRR is
CMRR=20log(Ad/(⃓Acm⃓))
where Ad is the differential gain and Acm is the common-mode gain

The common-mode rejection ratio (CMRR) of a differential amplifier (or other device) is the tendency of the devices to reject the input signals common to both input leads. A high CMRR is important in applications where the signal of interest is represented by a small voltage fluctuation superimposed on a (possibly large) voltage offset, or when relevant information is contained in the voltage difference between two signals.
The CMRR is defined as the ratio of the powers of the differential gain over the common-mode gain, measured in positive decibels.
The equation of CMRR is
CMRR=20log(Ad/(⃓Acm⃓))
where Ad is the differential gain and Acm is the common-mode gain