Adversary

Individual, group, organization, or government that conducts or has the intent to conduct detrimental activities. 

See CNSSI 4009-2015 (NIST SP 800-30 Rev. 1) under Adversary (DHS Risk Lexicon) for more information. 
 
An entity that is not authorized to access or modify information, or who works to defeat any protections afforded the information. 

See NIST SP 800-107 Rev. 1 under Adversary for more information.