Hardware, firmware, or software that is intentionally included or inserted in a system for a harmful purpose.
Sources:
CNSSI 4009-2015 under malicious logic from IETF RFC 4949 Ver 2

  See Malicious Code.
Sources:
NIST SP 800-12 Rev. 1 under Malware
NIST SP 800-128 under Malware

  Software or firmware intended to perform an unauthorized process that will have adverse impact on the confidentiality, integrity, or availability of an information system. A virus, worm, Trojan horse, or other code-based entity that infects a host. Spyware and some forms of adware are also examples of malicious code.
Sources:
CNSSI 4009-2015 under malicious code
NIST SP 1800-21B under Malware
NIST SP 800-128 under malicious code
NISTIR 7621 Rev. 1 under Malware

  A program that is inserted into a system, usually covertly, with the intent of compromising the confidentiality, integrity, or availability of the victim’s data, applications, or operating system or of otherwise annoying or disrupting the victim.
Sources:
NIST SP 800-137 under Malware
NIST SP 800-28 Version 2 under Malware
NIST SP 800-45 Version 2 under Malware

  See malicious code and malicious logic.
Sources:
CNSSI 4009-2015

  Software designed and operated by an adversary to violate the security of a computer (includes spyware, virus programs, root kits, and Trojan horses).
Sources:
NIST SP 800-152 under Malware

  Software or firmware intended to perform an unauthorized process that will have adverse impact on the confidentiality, integrity, or availability of a system. A virus, worm, Trojan horse, or other code-based entity that infects a host. Spyware and some forms of adware are also examples of malicious code.
Sources:
NIST SP 800-12 Rev. 1 under Malicious Code
NIST SP 800-172 under malicious code
NIST SP 800-172A under malicious code
NIST SP 800-171 Rev. 2 under malicious code

  A program that is written intentionally to carry out annoying or harmful actions, which includes Trojan horses, viruses, and worms.
Sources:
NIST SP 800-28 Version 2 under Malicious Code

  A virus, worm, Trojan horse, or other code-based malicious entity that successfully infects a host.
Sources:
NIST SP 800-61 Rev. 2 under Malware

  A program that is covertly inserted into another program with the intent to destroy data, run destructive or intrusive programs, or otherwise compromise the confidentiality, integrity, or availability of the victim’s data, applications, or operating system.
Sources:
NIST SP 800-83 Rev. 1 under Malware

  See Malicious malicious Ccode.
Sources:
NIST SP 800-128

  Software or firmware intended to perform an unauthorized process that will have adverse impact on the confidentiality, integrity, or availability of an information system. A virus, worm, Trojan horse, or other code-based entity that infects a host.
Sources:
NIST SP 800-82r3 under Malware from NIST SP 800-53 Rev. 5 - adapted

  Software or firmware intended to perform an unauthorized process that will have adverse impact on the confidentiality, integrity, or availability of an information system. A virus, worm, Trojan horse, or other code-based entity that infects a host.  Spyware and some forms of adware are also examples of malicious code.
Sources:
NIST SP 800-128 under Malicious Code

  Software or firmware intended to perform an unauthorized process that will have adverse impacts on the confidentiality, integrity, or availability of a system. A virus, worm, Trojan horse, or other code-based entity that infects a host. Spyware and some forms of adware are also examples of malicious code.
Sources:
NIST SP 800-53 Rev. 5 under malicious code

  A program that is inserted into a system, usually covertly, with the intent of compromising the confidentiality, integrity, or availability of the victim’s data, applications, or operating system.
Sources:
NIST SP 1800-10B under Malware from NIST SP 800-111
NIST SP 1800-25B under Malware from NIST SP 800-111
NIST SP 1800-26B under Malware from NIST SP 800-111

  An application that is covertly inserted into another piece of software (e.g., operating system, application) with the intent to steal or destroy data, run destructive or intrusive programs, or otherwise compromise the confidentiality, integrity, or availability of the victim’s data, applications, or operating system.
Sources:
NIST SP 800-167 from NIST SP 800-83 Rev. 1 - adapted