The ability to maintain required capability in the face of adversity.
Sources:
NIST SP 800-160 Vol. 2 Rev. 1 from System Engineering Handbook

  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:
NIST SP 800-39 under Information System Resilience

  The ability to prepare for and adapt to changing conditions and withstand and recover rapidly from disruption. Resilience includes the ability to withstand and recover from deliberate attacks, accidents, or naturally occurring threats or incidents.
Sources:
NIST SP 800-160 Vol. 2 Rev. 1 from OMB Circular A-130 (2016)
NIST IR 8441 from PPD-21

  The ability to prepare for and adapt to changing conditions and withstand and recover rapidly from disruptions. Resilience includes the ability to withstand and recover from deliberate attacks, accidents, or naturally occurring threats or incidents.
Sources:
CNSSI 4009-2015 from PPD-21
NIST SP 800-37 Rev. 2 from CNSSI 4009-2015
NIST IR 8323r1 from PPD-21
NIST IR 8401 from PPD-21

  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

  The ability of an information system to operate under adverse conditions or stress, even if in a degraded or debilitated state, while maintaining essential operational capabilities, and to recover to an effective operational posture in a time frame consistent with mission needs.
Sources:
NIST SP 800-53 Rev. 5 from OMB Circular A-130 (2016)

  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