Actions taken in cyberspace that create noticeable denial effects (i.e., degradation, disruption, or destruction) in cyberspace or manipulation that leads to denial that appears in a physical domain, and is considered a form of fires.
Sources:
CNSSI 4009-2022 under cyber attack from DoD JP 3-12 - under "cyberspace 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

  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 800-82r3 under attack

  Any kind of malicious activity that attempts to collect, disrupt, deny, degrade, or destroy information system resources or the information itself.
Sources:
NIST IR 8323r1 under attack
NIST IR 8401 under attack from CNSSI 4009-2022
NIST IR 8441 under attack from NIST SP 800-30 Rev. 1