A lawyer or attorney in the USA charged with investigation and prosecution of criminal charges on behalf of the government.
A specialized criminal law attorney who is on salary with the Government and who receives and reviews law enforcement reports on suspected criminal activity, supervises or directs investgations, and who then "translates" allegations of fact into formal charging documents, and then supervises its path to adjudication, including conduct of the government's case before a Grand Jury (if the charges set-out a felony), and jury trial.
The district attorney has the authority to decline to prosecute.
In other common law jurisdictions, they are the equivalent of a Crown prosecutor or Crown counsel.
The litigation adversary of a district attorney is the defence attorney or the public defender.
In some of the United States, this is an elected office; candidates state on the record, or "negotiate" their positions on crime, as it were, and the electorate chooses between those expressed positions. International observers are mixed in their opinions on whether or not it is appropriate for a prosecutor to be subject to the alliances necessary to any election campaign.
Because the attorneys are assigned to judicial territorial districts, they are usually assisted by a considerable staff of assistants. Usually, the actual District Attorney for any particular district, only handles a handful of cases - if any - delegating to her/his army of assistant district attorneys the vast majority of prosecutions. For example, in New York City (Manhattan), there is 1 district attorney and 500 assistant district attorneys. Philadelphia has over 600 ADAs.
In order to insulate these lawyers from corruption, they are immune from liability for actions they take, or decisions they make, within the scope of their official duties.
It is the responsibility of the district attorney act as chief law enforcement officer, to assume leadership in the prevention of crime by prosecuting criminals and accommodating the rights of crime victims.
References or Further Reading:
Los Angeles County District Attorney Office at da.co.la.ca.us
The National District Attorney Association at ndaa.org
Manhattan District Attorney, at manhattanda.org
Philadelphia Office of the District Attorney at phila.gov/districtattorney