A special prosecutor (or special counsel or independent counsel) is a lawyer appointed to investigate and possibly prosecute a specific legal case of potential wrongdoing for which a conflict of interest exists for the usual prosecuting authority. For example, the investigation of an allegation against a sitting president or attorney-general might be handled by a special prosecutor rather than an ordinary prosecutor, who would otherwise be in the position of investigating their own boss. Investigations into others connected to the government but not in a position of direct authority over the prosecutor, such as cabinet secretaries or election campaigns, have also been handled by special prosecutors.
The term is not specific to the United States, nor to the federal government. According to Harriger, the concept originates in state law: "state courts have traditionally appointed special prosecutors when the regular government attorney was disqualified from a case, whether for incapacitation or interest." While the most prominent special prosecutors have been those appointed since the 1870s to investigate presidents and those connected to them, the term can also be used to refer to any prosecutor appointed to avoid a conflict of interest or appearance thereof. For example, because district attorneys' offices work closely with police, some activists argue that cases of police misconduct at the state and local level should be handled by "special prosecutors". Wikipedia