1) in 2006, Gartner coined the term Operational Technology.

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"

According to the ISA (International Society of Automation), ACS is defined as a collection of processes, personnel, hardware, and software that influence 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.
Originally this acronym was IACS but the "Industrial" descriptor was removed because automation and control systems also exist in many non-industrial environments. The ACS and IACS terms should therfore be treated as synonyms.

3) Unfortunately, neither the ACS or OT definition refers to a system architecture or the responsible professional groups.

As a result, it is challenging to compare these terms, as neither identify where they fit in the Enterprise Architecture nor even who is responsible for them. Unfortunately, the the two terms are sometimes used as synonyms. However, this may be untrue, misleading, or even dangerous, as IACS may be better suited for dangerous industrial facilities. As stated above, "A principal driver of the adoption of the OT term was that the nature of operational technology platforms had evolved from bespoke proprietary systems to complex software portfolios that rely on IT infrastructure." Thus, ACS and OT are complementary approaches and, in some cases, may even be opposites.

By contrast, ISA95 (Enterprise Integration) , ISA99 (cybersecurity) and ISA 84 (Safety Instrumented Systems) began in process industries where "bespoke proprietary systems" (such as DCS, SCADA, PLCs, etc.) were the norm. Since these vendors were used to working with dangerous processes, they implemented secure architectures implemented with high-integrity thoroughly-tested hardware and software. According to the Gartner definition, it was these secure systems that OT wished to displace. ISA also clearly established who is responsible for the safe design, operation, and maintenance of the ACS. These professions include Control Engineers (for control strategies and Operator HMIs), Electrical Engineers (for hazardous area safety), and Process Engineers (for Optimization Strategies).

It is true, however, that better interfaces between ACS and IT are desirable, and an "OT approach" may sometimes be preferable. However, PERA proposes that this should be done with "interfacing rather than "integration". At the very least, the designers should be clear about which approach they are taking, and why.
See the following FAQ entitled "What is ACS, IT, OT, and IoT" for more detailed discussion.