A correction factor that is smaller than 1 if the sphericity assumption in an ANOVA is violated. The factor is multiplied with F, making the test more conservative. It is a method of correcting the degrees of freedom of the within-subject F-tests in the analysis of variance of repeated measures or longitudinal data so as to allow for departures of the variance-covariance matrix from the assumption of sphericity.
See Analysis of variance (ANOVA), Hunyh-Feldt epsilon, Mauchly test, Repeated measures Sphericity, Sphericity assumption