APA Dictionary of Psychology

a distribution in which the noncentrality parameter is not equal to zero: In statistical significance testing, such a distribution is obtained when the null hypothesis under test is false. The noncentral version of a distribution has a different mean, skewness, and variance (among other properties) than its corresponding central distribution, as well as a larger proportion of numbers beyond the critical values. For example, the chi-square distribution, F distribution, and t distribution all have corresponding noncentral versions signified by a noncentrality parameter that is not equal to zero.