APA Dictionary of Psychology

n. in factor analysis, that part of the variance of a variable that it does not share with any other variable in the system. It is given as 1 – h_j², where h_j² is the communality of the jth variable. Each of the observed variables in the data set being analyzed can be expressed as a combination of a common factor shared among all variables plus a unique factor associated with a measurement error or another specific, individual source of variation.