APA Dictionary of Psychology

the square root of the sum of the squares of a set of values divided by the number of values. For a set of values x₁, x₂, … x_n, the root mean square value is
√/[(x₁² + x₂² + … x_n²)/n]
and describes the average size of the values in the set. In the physical sciences the term is used as a synonym for standard deviation under certain circumstances.