Critical infrastructure refers to the assets that provide vital services required to sustain the public health and safety of a population and protect the security and economic well-being of a nation.
In Presidential Policy Directive 21 (PPD-21), the United States designated the following 16 critical infrastructure sectors: Chemical; Commercial Facilities; Communications; Critical Manufacturing; Dams; Defense Industrial Base; Emergency Services; Energy; Financial Services; Food and Agriculture; Governmental Facilities; Healthcare and Public Health; Information Technology; Nuclear Reactors, Materials, and Waste; Transportation Systems; and Water and Wastewater Systems.
Supervisory control and data acquisition (SCADA) systems as well as industrial control systems (ICS) are in the Information Technology sector. These are critical systems that must be protected, as they can affect a wide range of services, from the flow of electricity to the operations and control of commercial transportation.
References
US Department of Homeland Security (n.d.). Critical infrastructure sectors. https://www.dhs.gov/what-critical-infrastructure
White House. (2013). Presidential Policy Directive 21: Critical infrastructure security and resilience. US Government Printing Office.