The mean of the squares of the differences between a set of data values and some fixed central value (usually the mean); the variance of a set of values; (formerly) †the standard deviation of a set of values (obsolete).
Origin
1960s.
Definition of mean-square error in US English:
mean-square error
noun
Statistics
The mean of the squares of the differences between a set of data values and some fixed central value (usually the mean); the variance of a set of values; (formerly) †the standard deviation of a set of values (obsolete).