Variance Components (in Mixed Model ANOVA)

The term variance components is used in the context of experimental designs with random effects , to denote the estimate of the (amount of) variance that can be attributed to those effects. For example, if one were interested in the effect that the quality of different schools has on academic proficiency, one could select a sample of schools to estimate the amount of variance in academic proficiency (component of variance) that is attributable to differences between schools.
See also, Analysis of Variance and Variance Components and Mixed Model ANOVA/ANCOVA .