(4) is the error. In practical situations, the output values obtained are subject to variability, that...

(4) ε is the error. In practical situations, the output values obtained are subject to variability, that is, under the same conditions, it is unlikely that the same output values will be obtained due to measurement error, analysis errors, or the like. These errors mean that the predicted value given by g will be different from the measured values. The error is often assumed to be a stochastic, that is, random, variable whose values follow the normal distribution. Pg45