APA Dictionary of Psychology

a statistical model that acknowledges different levels in the data, such that individuals or entities within each level have correlated scores. The model enables a researcher to test hypotheses about cross-level effects and partition the variance and covariance components among levels. For example, hierarchical linear models often are used in educational research since they can account for the fact that students within a classroom will behave similarly, as will classrooms within the same school, and so on up the hierarchy of levels. Also called multilevel model.