1) In 2006, Gartner coined the term Operational Technology (OT).

According to Wikipedia: "The term operational technology as applied to industrial control systems was first published in a research paper from Gartner in May 2006 (Steenstrup, Sumic, Spiers, Williams) and presented publicly in September 2006 at the Gartner Energy and Utilities IT Summit.[2] Initially, the term was applied to power utility control systems, but over time was adopted by other industrial sectors and used in combination with IoT.[3] A principal driver of the adoption of the term was that the nature of operational technology platforms had evolved from bespoke proprietary systems to complex software portfolios that rely on IT infrastructure."

2) "ACS" stands for "Automation and Control Systems.

Originally this acronym was IACS (pronounced eye-AX), but the "Industrial" descriptor was deleted because automation and control systems also exist in many non-industrial environments. Automation and Control Systems (ACS) is also defined as the preferred term by IEC (International Electrotechnical Commission) since IACS did not translate well. The ACS acronym is therefore always preferred, unless "industrial" is used as an adjective describing the Automation and Controls System (as in "Industrial ACS", or IACS).
According to the latest committee draft (CD) of IEC/ISA 62443-1-1, ACS is defined as "a collection of processes, personnel, hardware, and software that influences the safe, secure, and reliable operation of an industrial process"; essentially encompassing all components involved in automating an industrial operation, including sensors, controllers, actuators, and communication networks.
ISA has also established an extensive set of requirements and certifications for professionals who design and approve ACS systems.

3) "IT" is widely understood as "Information Technology."

This includes providing and supporting the infrastructure required to process this information. However, the definition of IT does not include "safe, secure, and reliable operation of an industrial process". The confusion, therefore, occurs when IT infrastructure is used to directly or indirectly control an ACS in a hazardous facility. There is no IT organization that defines requirements or certifies professionals to design or support hazardous Automation and Control Systems (ACS). This is not a problem for non-hazardous automation like door locks or scanners in a commercial building automation system (BMS). However, if the BMS is for a hospital, and includes industrial backup generators, boilers, and hazardous materials, ACS infrastructure and control systems will be necessary.

4) It is evident that a working definition of OT systems is required.

This definition must clearly establish how OT differs from IT and ACS. This is discussed at length in two Micro Learning Modules (MLM-014-A) and (MLM-014B) that may be freely downloaded. The conclusions reached include the following definition of OT that is used on the PERA website and associated materials.

It is historically true that better interfaces are needed between ACS and IT at both technical and human levels. PERA proposes that technical interfaces must be clearly specified as described in MLM-034-A rather than "integrated" or "converged". At a human level, it is hoped that the above definition will help bridge the IT-ACS gap by providing a firm foundation at both ends.