PID = Proportional Integral Derivative, I think, it's a type of feedback loop used for digital building and HVAC control systems, more specifically, the turd of a system that's known as Johnson Controls Metasys. How I love to hate writing that code.
Wow AOD, I dunno how it was writing Metasys code, but it was a charm of an EMS if set up properly...had it in the buildings I used to work at, now using Trane Tracer Summit, which we have V.17, (the latest) which is still primitive compared to Metasys as far as trending capabilities are concerned. At that facility, we converted the pneumatics over as we built/rebuilt areas. Honeywell Deltanet had funny cartoon-like graphics.