APA Dictionary of Psychology

in statistical hypothesis testing, a population parameter of secondary interest that must be accounted for in order to obtain an estimated value for a parameter of primary interest. For example, assume a researcher wishes to determine whether older schoolchildren differ in their mathematics ability. They might administer a math test to a sample of children at a particular school and use that data to estimate age-based means for all children. If, however, the researcher needed to calculate the variance of the scores before attempting to determine the means, the former would be a nuisance parameter.