System and assets, whether physical or virtual, so vital to the U.S. 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.
Sources:
CNSSI 4009-2015

  System 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.
Sources:
NIST SP 800-30 Rev. 1 under Critical Infrastructure

  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.
Sources:
NIST SP 800-53 Rev. 5 from PL 107-56 (Patriot Act)

  Essential services and related assets that underpin American society and serve as the backbone of the nation's economy, security, and health.
Sources:
NISTIR 8183 under Critical Infrastructure from DHS - Unknown Source, National Cybersecurity & Communications Integration Center
NISTIR 8183 Rev. 1 under Critical Infrastructure from DHS
NISTIR 8183A Vol. 1 under Critical Infrastructure from DHS
NISTIR 8183A Vol. 2 under Critical Infrastructure from DHS
NISTIR 8183A Vol. 3 under Critical Infrastructure from DHS

  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 cybersecurity, national economic security, national public health or safety, or any combination of those matters.
Sources:
NIST Cybersecurity Framework Version 1.1 under Critical Infrastructure