PID stands for Proportional-Integral-Derivative, and it is a type of feedback loop controller used to maintain a desired temperature. In the case of an espresso machine, the PID controller monitors the temperature of the water in the boiler and makes adjustments as needed to keep it at the correct temperature.

PID actually stands for Proportional Integral Derivative. In terms of coffee, it simply means that the espresso machine is capable of constantly monitoring the water temperature and making small adjustments to keep it at the set temperature. Without a PID controller, an espresso machine works a lot like a thermostat.