- A signal indicating an error.
- A magnitude which is proportional to the difference between a desired or reference value, and the actual or measured value. In radars, for instance, it may refer to the difference between an actual position and a desired position. When said magnitude is a voltage, also called error voltage (1).
- In an automatic control system, a signal whose magnitude is proportional to the difference between the actual operating quantity and a quantity which would be within the specified parameters. Such a signal is utilized for automatic correction. When said magnitude is a voltage, also called error voltage (2).