What is the term used for the difference between the set point and the actual value of the process variable?

The term that represents the difference between the set point and the actual value of the process variable is referred to as "Error." It quantifies the discrepancy that exists within a control system between what is desired (the set point) and what is currently being measured (the actual value). This concept is fundamental in control engineering, as it allows engineers and operators to assess how well a system is performing against its intended performance goals.

Understanding this definition is crucial, as it helps in diagnosing system performance and implementing control measures to minimize this error, thus enhancing system stability and performance. The term "offset" may also describe a consistent deviation but does not capture the dynamic nature of the difference as effectively as "error." Additionally, "drift" refers to the gradual change in measurement or control parameters over time, while "deviation" can refer to any variation from a standard, making "error" the most precise choice in this context.