PID

PID stands for a number of things, including:

Principal ideal domain, in abstract algebra, an integral domain in which every ideal is principal
Proportional-Integral-Derivative controller, a feedback loop component in control theory and industrial control applications
Pelvic inflammatory disease, an infection of the female uterus, fallopian tubes, and/or ovaries
Police Intelligence Department, a staff department of the Singapore Police Force
Political Intelligence Department, a British clandestine body created to produce and disseminate propaganda during WWII
process identifier, a number used by Unix kernels to identify a process
Pathways Into Darkness, a 1993 video game by Bungie Studios
Paul Is Dead, a rumor in the 1960s that led fans to believe that Paul McCartney was actually dead
Packet ID, used to identify audio/video streams in DVB broadcasting
Pid is a nick-name, frequently a short form of Stupid
Primary immune deficiency
Prolapsed intervertebral disc, commonly called a "slipped disc"
Photo Ionization Detector
Physician Induced Demand
Project Information Document
Personal Integrating/Indicating Dosemeter used to indicate amount of radiation a person has received
Project Initiation Document defines all major aspects of the project and forms the basis for its management and the assessment of overall success