Hello, could you tell us more. I can say there is a random error variable in mathematical models. To start, let say (generally speaking) it represents the difference between the observed error values and the error values predicted by the model. You can look at simple linear regression models dependent (Y) vs independent variable (X), Markov Chain, Ito process with Brownian motion, chaos, stockastic control system, control system with noise, market and forex, etc. If you have a specific case in mind, feel free to throw it in the discussion. Best regards