Programmable systems or devices that interact with the physical environment (or manage devices that interact with the physical environment). These systems/devices detect or cause a direct change through the monitoring and/or control of devices, processes, and events. Examples include industrial control systems, building management systems, fire control systems, and physical access control mechanisms.
Source(s):
NIST SP 800-37 Rev. 2 under operational technology

  Programmable systems or devices that interact with the physical environment (or manage devices that interact with the physical environment). These systems or devices detect or cause a direct change through the monitoring or control of devices, processes, and events. Examples include industrial control systems, building management systems, fire control systems, and physical access control mechanisms.
Source(s):
NIST SP 800-160 Vol. 2 Rev. 1 under operational technology
NIST SP 800-160 Vol. 2 [Superseded] under operational technology

  The hardware, software, and firmware components of a system used to detect or cause changes in physical processes through the direct control and monitoring of physical devices.
Source(s):
NIST SP 800-172 under operational technology

  Programmable systems or devices that interact with the physical environment (or manage devices that interact with the physical environment).
Source(s):
NIST SP 1800-10B under Operational Technology from NIST SP 800-37 Rev. 2 - Adapted

  Hardware and software that detects or causes a change through the direct monitoring and/or control of physical devices, processes and events in the enterprise.
Source(s):
NISTIR 8183 under Operational technology from Gartner.com
NISTIR 8183 Rev. 1 under Operational technology from Gartner.com
NISTIR 8183A Vol. 1 under Operational technology from Gartner.com
NISTIR 8183A Vol. 2 under Operational technology from Gartner.com
NISTIR 8183A Vol. 3 under Operational technology from Gartner.com