Critical infrastructure

Critical infrastructure is formally defined (originally under the USA PATRIOT Act, and carried into Presidential Policy Directive 21) as systems and assets, physical or virtual, so vital to the United States that their incapacity or destruction would have a debilitating impact on national security, the national economy, public health, or public safety. In practice, this covers the systems most people never think about until they stop working: the power grid, water treatment, financial systems, hospitals, and more.

Note: PPD-21 was formally superseded by a White House National Security Memorandum on Critical Infrastructure Security and Resilience, issued April 30, 2024. As of this writing, CISA's own sector pages still reference PPD-21 for sector designations, so both should be cited for full accuracy.