also as disturbance , residual , error term , error ; it denotes the difference between the actual value of y and the predicted value of y for population data( E ) or for sample data( e ).
Note1: It is a random(stochastic) variable that has well-defined probabilistic properties.
Note2: it may well represent all those factors that affect dependent variable but are not taken into account explicitly.
see random short-range variation in spatial context;
see probabilistic model for numerical definition