Virus definition in Computer Security terms:
Definition(s): A computer program that can copy itself and infect a computer without permission or knowledge of the user. A virus might corrupt or delete data on a computer, use e-mail programs to spread itself to other computers, or even erase everything on a hard disk. See malicious code.
Source(s): CNSSI 4009-2015
A program that replicates itself by attaching to other programs or files, where it hides until activated.
Source(s): NIST SP 800-28 Version 2
A computer program containing a malicious segment that attaches itself to an application program or other executable component.
Source(s): NIST SP 800-47
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.
Source(s): NIST SP 800-82 Rev. 2 (RFC 4949)
reference: CSRC Glossary