A security event, or a combination of multiple security events, that constitutes a security incident in which an intruder gains, or attempts to gain, access to a system or system resource without having authorization to do so.
Sources:
CNSSI 4009-2015
from
IETF RFC 4949 Ver 2
See intrusion.
Sources:
CNSSI 4009-2015
under penetration
any set of actions that attempts to compromise the integrity, confidentiality, or availability of a resource
Sources:
NIST SP 800-35
under Intrusion