What is Special Prosecutor?

Legal Definition
A special prosecutor generally is a lawyer from outside the government appointed by an attorney general or, in the United States, by Congress to investigate a government official for misconduct while in office. A reasoning for such an appointment is that the governmental branch or agency may have political connections to those it might be asked to investigate. Inherently, this creates a conflict of interest and a solution is to have someone from outside the department lead the investigation. The term "special prosecutor" may have a variety of meanings from one country to the next, from one government branch to the next within the same country, and within different agencies within each government branch. Critics of the use of special prosecutors argue that these investigators act as a "fourth branch" to the government because they are not subject to limitations in spending, nor do they have deadlines to meet.
-- Wikipedia
Legal Definition
the name of an attorney that is retained by the prosecuting lawyer to assist him in trying a case. See special counsel.