A form of automatic tuning of the PID terms available on most controllers. This type of tuning is usually carried when the controller is at or close to ambient conditions.
The controller drives the process part-way towards the target setpoint (typically 50-75% from the start value to setpoint), then induces a temporary disturbance by turning the control outputs off for a short period. The process response is analyzed and the correct tuning terms are calculated.
This type of automatic tuning requires sufficient distance between the process variable and the setpoint when it is used and can cause an over-shoot of the setpoint during the test. If this is a problem for the application, consider using tune at setpoint if it is available.