The ability of an information system to continue to: (i) operate under adverse conditions or stress, even if in a degraded or debilitated state, while maintaining essential operational capabilities; and (ii) recover to an effective operational posture in a time frame consistent with mission needs.
Sources:
CNSSI 4009-2015 from NIST SP 800-39
NIST SP 800-39 under Information System Resilience

  The ability of an information system to continue to operate while under attack, even if in a degraded or debilitated state, and to rapidly recover operational capabilities for essential functions after a successful attack.
Sources:
NIST SP 800-30 Rev. 1 under Information System Resilience

  The ability to quickly adapt and recover from any known or unknown changes to the environment through holistic implementation of risk management, contingency, and continuity planning.
Sources:
NIST SP 800-34 Rev. 1 under Resilience

  See Information System Resilience.
Sources:
NIST SP 800-39 under Resilience

  The ability to continue to: (i) operate under adverse conditions or stress, even if in a degraded or debilitated state, while maintaining essential operational capabilities; and (ii) recover to an effective operational posture in a time frame consistent with mission needs.
Sources:
NIST SP 800-137 under Resilience from NIST SP 800-39 - Adapted

  can also be defined as the adaptive capability of an organization in a complex and changing environment.
Sources:
NISTIR 8074 Vol. 2 under Resilience from ASIS SPC.1-2009

  The ability to reduce the magnitude and/or duration of disruptive events to critical infrastructure. The effectiveness of a resilient infrastructure or enterprise depends upon its ability to anticipate, absorb, adapt to, and/or rapidly recover from a potentially disruptive event.
Sources:
NISTIR 8074 Vol. 2 under Resilience from Critical Infrastructure Resilience Final Report and Recommendations