An undesired change in a variable which adversely affects the value of a controlled variable is called a ______.

The term that describes an undesired change in a variable which negatively impacts the value of a controlled variable is known as a disturbance. Disturbances are external or internal influences that can disrupt the desired operating conditions of a control system. They affect the process without being directly related to the control inputs and can arise from various sources such as environmental changes, fluctuations in setpoint, or variations in system dynamics.

By identifying disturbances, control systems can be designed to respond effectively and maintain stability and performance. Understanding disturbances is crucial for control system technicians as it aids in troubleshooting and optimizing control strategies to minimize their adverse effects.