Traditionally, an agent provocateur is a person employed by the police or other entity to act undercover to entice or provoke another person to commit an illegal act. More generally, the term may refer to a person or group that seeks to discredit or harm another by provoking them to commit a wrong or rash action.
As a known tool to prevent infiltration by agents provocateurs, the organizers of large or controversial assemblies may deploy and coordinate demonstration marshals, also called stewards.