Nolle prosequi is Latin for “no wish to prosecute.” As its Latin translation implies, a nolle prosequi (or “nol pros”) is a notice from the prosecutor stating that he is not willing to proceed against a criminal defendant. The prosecutor is essentially abandoning the action and, consequently, the charge or charges against the defendant are being dropped.
A nolle prosequi usually takes the form of a written motion submitted to the court by the prosecutor. Such a motion can be filed at any time after arraignment or before sentencings.
According to Rule 16(a) of the Massachusetts Rules of Criminal Procedure:
A prosecuting attorney may enter a nolle prosequi of pending charges at any time prior to the pronouncement of sentence or the imposition of probation or the entry of an order of continuance without a finding. A nolle prosequi shall be accompanied by a written statement, signed by the prosecuting attorney, setting forth the reasons for that disposition.
The notes for Rule 16 add that,
The decision to enter a nolle prosequi as to all or any distinct part of pending charges is discretionary with the prosecuting attorney. Power to enter a nolle prosequi is absolute in the prosecuting officer from the return of the indictment up to the beginning of trial, except possibly in instances of scandalous abuse of the authority.