random error

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