A solid-state control system that has a user-programmable memory for storing instructions for the purpose of implementing specific functions such as I/O control, logic, timing, counting, three mode (PID) control, communication, arithmetic, and data and file processing.
Sources:
NIST SP 800-82 Rev. 2 under Programmable Logic Controller (PLC) from The Automation, Systems, and Instrumentation Dictionary
NISTIR 8183 from NIST SP 800-82 Rev. 2
NISTIR 8183 Rev. 1 from NIST SP 800-82 Rev. 2
NISTIR 8183A Vol. 1
NISTIR 8183A Vol. 2
NISTIR 8183A Vol. 3

  A small industrial computer originally designed to perform the logic functions executed by electrical hardware (relays, switches, and mechanical timer/counters). PLCs have evolved into controllers with the capability of controlling complex processes, and they are used substantially in SCADA systems and DCS. PLCs are also used as the primary controller in smaller system configurations. PLCs are used extensively in almost all industrial processes.
Sources:
NIST SP 800-82 Rev. 2 under Programmable Logic Controller (PLC)