Type of malicious software that propagates itself by modifying other programs to include a possibly changed copy of itself and that is executed when the infected program is invoked. Note: A virus often causes damage and may be triggered by some event such as the occurrence of a predetermined date.
Sources:
CNSSI 4009-2022 from ISO/IEC 2382:2015
NIST SP 800-12 Rev. 1 under Virus from CNSSI 4009

  A program that replicates itself by attaching to other programs or files, where it hides until activated.
Sources:
NIST SP 800-28 Version 2 under Virus

  A hidden, self-replicating section of computer software, usually malicious logic, that propagates by infecting (i.e., inserting a copy of itself into and becoming part of) another program. A virus cannot run by itself; it requires that its host program be run to make the virus active.
Sources:
NIST SP 800-82r3 from RFC 4949 - adapted