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. 

See NIST SP 800-82 Rev. 2 under Virus (RFC 4949) for more information. 
 
A computer program containing a malicious segment that attaches itself to an application program or other executable component. 

See NIST SP 800-47 under Virus for more information. 
 
A program that replicates itself by attaching to other programs or files, where it hides until activated. 

See NIST SP 800-28 Version 2 under Virus for more information. 
 
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 the following under Virus for more information: 
CNSSI 4009-2015.
NIST SP 800-12 Rev. 1  (CNSSI 4009).