Any kind of malicious activity that attempts to collect, disrupt, deny, degrade, or destroy information system resources or the information itself.
Sources:
CNSSI 4009-2015
NIST SP 800-12 Rev. 1 under Attack from CNSSI 4009
NIST SP 800-30 Rev. 1 under Attack from CNSSI 4009
NIST IR 8323r1 from CNSSI 4009-2015
NIST IR 8401
NIST IR 8441 from NIST SP 800-30 Rev. 1

  An attack, via cyberspace, targeting an enterprise’s use of cyberspace for the purpose of disrupting, disabling, destroying, or maliciously controlling a computing environment/infrastructure; or destroying the integrity of the data or stealing controlled information.
Sources:
NIST SP 1800-10B under Cyber Attack from NIST SP 800-30 Rev. 1
NIST SP 800-30 Rev. 1 under Cyber Attack from CNSSI 4009
NIST SP 800-39 under Cyber Attack from CNSSI 4009

  An attempt to gain unauthorized access to system services, resources, or information, or an attempt to compromise system integrity, availability, or confidentiality.
Sources:
NIST SP 800-82 Rev. 2 under Attack from CNSSI 4009

  The realization of some specific threat that impacts the confidentiality, integrity, accountability, or availability of a computational resource.
Sources:
NIST SP 800-28 Version 2 under Attack

  An unauthorized entity’s attempt to fool a verifier or RP into believing that the unauthorized individual in question is the subscriber.
Sources:
NIST SP 800-63-3 under Attack