A correction factor that is smaller than 1 if the sphericity assumption in a repeated measures ANOVA is violated. The factor is multiplied with F, making the test more conservative. This factor is comparable to the Greenhouse-Geisser epsilon, but it tends to be less conservative.
See Analysis of variance (ANOVA), Greenhouse-Geisser epsilon, Mauchly test, Repeated measures analysis of variance, Sphericity, Sphericity assumption