Intelligence term designating:

1) intelligence operations carried out clandestinely by operatives disguised as foreign nationals [In Russian, “nelegal’naya razvedka,” or “illegal intelligence”] – as opposed to “legal” [Russian “legal’naya”] operations, the term for intelligence carried out under a diplomatic, journalistic or any other cover. Illegal operations were widely used by all branches of Soviet intelligence beginning in the early 1920s. In different periods, they were also used by other nations with strong intelligence services, such as Great Britain, Japan, Israel, Germany and China;

2) an intelligence agent operating clandestinely under the alias of a foreign national, without any cover, whether diplomatic, journalistic or other [In Russian, “nelegal,”  or “illegal” agent or operative].