1. Systems and assets, whether physical or virtual, so vital to the United States that the incapacity or destruction of such systems and assets would have a debilitating impact on security, national economic security, national public health or safety, or any combination of those matters. Critical infrastructure can be owned and operated by both the public and private sector. [Critical Infrastructure Protection Act of 2001, 42 U.S.C. 519c(e)] (CNSSI 4009, Adapted).
Source: Starting Point: U.S. Election Systems as Critical Infrastructure, U.S. Election Assistance Commission, https://www.eac.gov/sites/default/files/eac_assets/1/6/starting_point_us_election_systems_as_Critical_Infrastructure.pdf; Explore Terms: A Glossary of Common Cybersecurity Terminology, National Initiative for Cybersecurity Careers and Studies (NICCS), https://niccs.us-cert.gov/about-niccs/glossary; NSTAC Report to the President on a Cybersecurity Moonshot, National Security Telecommunications Advisory Committee (NSTAC), https://www.dhs.gov/sites/default/files/publications/NSTAC_CyberMoonshotReport_508c.pdf; The State and Local Election Cybersecurity Playbook, Defending Digital Democracy Project, Harvard Kennedy School Belfer Center for Science and International Affairs, https://www.belfercenter.org/sites/default/files/files/publication/StateLocalPlaybook%201.1.pdf; Source: U.S. Cyberspace Solarium Commission, March 2020, https://subscriber.politicopro.com/f/?id=00000170-c638-d8f7-a7f1-f63b33510000
2. Also, [within DoD] infrastructure deemed essential to DoD operations or the functioning of a critical asset. Nation's critical infrastructure and key resources, as set forth in the 2006 National Infrastructure Protection Plan (NIPP) includes the assets, systems, and networks, whether physical or virtual, so vital to the United States that their incapacitation or destruction would have a debilitating effect on security, national economic security, public health or safety, or any combination thereof. Key resources are publicly or privately controlled resources essential to the minimal operations of the economy and government.
Source: National Strategy for Homeland Security Terms & Definitions of Interest for DoD Counterintelligence Professionals, Office of the National Counterintelligence, https://www.dni.gov/files/NCSC/documents/ci/CI_Glossary.pdf
3. Infrastructure or part of the economy whose failure or breakdown would have enormous consequences on national security or the economic and/or social welfare of a nation. In Switzerland the following infrastructure has been defined as critical: energy and water supply, emergency and rescue services, telecommunications, transport and traffic, banks and insurance, government and public administration. In the information age their smooth running is increasingly dependent upon information and communication systems. Systems such as these are referred to as critical information infrastructures.
Source: Information Assurance Situation in Switzerland and Internationally, Reporting and Analysis Centre for Information Assurance MELANI, https://www.newsd.admin.ch/newsd/message/attachments/11945.pdf