Here we use the broader definition of linearity to mean constant dynamics. A linear control loop has a constant total loop dead time, constant primary and secondary time constants, and a constant open loop gain. This perspective reviews the sources of dynamics and causes of nonlinearity.
A significant part of the challenge in understanding process control is the proper use of terms to describe the dynamics in a control loop. Communication can be greatly improved by focusing on three key terms and adding a few words to more aptly describe the source of the term.