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 under attack
NIST IR 8323r1 under attack from CNSSI 4009-2015
NIST IR 8401 under attack

  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 from NIST SP 800-30 Rev. 1
NIST SP 800-30 Rev. 1 from CNSSI 4009
NIST SP 800-39 from CNSSI 4009

  Actions taken through the use of computer networks to disrupt, deny, degrade, or destroy information resident in computers and computer networks, or the computers and networks themselves. Note: Within DoD, Joint Publication 3-13, "Information Operations, " 27 November 2012 approved the removal the terms and definitions of computer network attack (CNA), computer network defense (CND), computer network exploitation, and computer network operations (CNO) from JP -1-02, "Department of Defense Dictionary of Military Terms and Associated Terms." This term and definition is no longer published in JP 1-02. This publication is the primary terminology source when preparing correspondence, to include policy, strategy, doctrine, and planning documents. The terms are no longer used in issuances being updated within DoD. JP 1-02, following publication of JP 3-12, "Cyberspace Operations" provides new terms and definitions such as cyberspace, cyberspace operations, cyberspace superiority, defensive cyberspace operation response action, defensive cyberspace operations, Department of Defense information network operations, and offensive cyberspace operations.
Sources:
CNSSI 4009-2015 under computer network attack (CNA)