A district attorney (D.A.) (or in New Jersey County Prosecutor and in Baltimore and Chicago State's Attorney (S.A.)) is the title of the local public official who represents the government in the prosecution of alleged criminals. The D.A. is the highest officeholder in the jurisdiction's legal department and supervises a staff of executive assistant district attorney and assistant district attorneys. It is typically an elected position, as in New York City, but this can vary from one jurisdiction to another. Therefore, it is common for DAs, unlike their subordinates, to factor in political considerations in their work.
In current canon, Jack McCoy is currently District Attorney.