Residual variance or unexplained variance is part of the variance of any residual. The other part is explained variance. In analysis of variance and regression analysis, residual variance is that part of the variance which cannot be attributed to specific causes.
The unexplained variance can be divided into two parts. First, the part related to random, everyday, normal, free will differences in a population or sample. Among any aggregation of data these conditions equal out. Second, the part that comes from some condition that has not be identified, but that is systematic. That part introduces a bias and if not identified can lead to a false conclusion.