A person who conducts investigations on behalf of the government; (now) specifically (US) a person who conducts criminal investigations and has arrest authority, especially as a member of the Federal Bureau of Investigation.
Origin
Late 19th century.
Definition of special agent in US English:
special agent
noun
A person who conducts investigations on behalf of the government; (now) specifically (US) a person who conducts criminal investigations and has arrest authority, especially as a member of the Federal Bureau of Investigation.