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 800-82r3
from
The Automation, Systems, and Instrumentation Dictionary
NISTIR 8183
from
NIST SP 800-82r3
NISTIR 8183 Rev. 1
from
NIST SP 800-82r3
NISTIR 8183A Vol. 1
NISTIR 8183A Vol. 2
NISTIR 8183A Vol. 3