Any kind of malicious activity that attempts to collect, disrupt, deny, degrade, or destroy information system resources or the information itself.
NIST SP 800-30
The realization of some specific threat that impacts the confidentiality, integrity, accountability, or availability of a computational resource.
NIST SP 800-28 Version 2
An attempt by an unauthorized individual to fool a Verifier or a Relying Party into believing that the unauthorized individual in question is the Subscriber.
NIST SP 800-63-2
An attempt to gain unauthorized access to system services, resources, or information, or an attempt to compromise system integrity, availability, or confidentiality.
NIST SP 800-82 Rev. 2