(1) The difference between the measured value of a quantity and the theoretical or defined value of that quantity: ε(error) ≡ y (measured) - y (theoretical). (2) The difference between an observed or calculated value of a quantity and the ideal or true value of that quantity. Logically, definitions (1) and (2) are distinctly different. In practice, they are equivalent except for the second definition's allowing calculated values to be used instead of measured values. (3) The difference between an approximate number and the correct number. (4) The difference between the theoretical or defined value of a quantity and the measured value of that quantity: ε (error) ≡ y (theoretical) - y (measured). This definition differs from the first definition only in changing the sign of the error.