A signal sent by an ideal transmitter or received by a receiver would have all constellation points precisely at the ideal locations, however various imperfections in the implementation (such as noise, low image rejection ratio, phase noise, carrier suppression, distortion, etc.)
or signal path cause the actual constellation points to deviate from the ideal locations.
The modulation error ratio is equal to the ratio of the root mean square (RMS) power (in Watts) of the reference vector to the power (in Watts) of the error.
It is defined in dB as: where Perror is the RMS power of the error vector, and Psignal is the RMS power of ideal transmitted signal.
MER is defined as a percentage in a compatible (but reciprocal) way: with the same definitions.