Nolle prosequi is a Latin phrase meaning "will no longer prosecute" or a variation on the same. It amounts to a dismissal of charges by the prosecution. Some states, like New York, for example, don't use the phrase. Rather, they simply use the term dismissal.
The prosecution invokes nolle prosequi or dismissal when it has decided to discontinue a prosecution or part of it. Lawyers and judges refer to the charges "nol prossed" or dismissed. The prosecution may nol pross all charges against the defendant or only some.
In some states, the prosecution must move to dismiss charges (often "in the interests of justice"), and the case doesn't end until the court grants the motion. In others, the prosecution can unilaterally dismiss charges.
A prosecutor might nol pross or dismiss charges for a variety of reasons, including:
Example: Wee is arrested for assault with a firearm. Officers take statements from two witnesses who say they saw him fire a gun at someone walking down the street. After reading the police reports, the local prosecutor decides to press charges. Weeks later, officers find the weapon underneath a garbage dumpster near the place of the shooting. Forensic testing reveals the fingerprints of someone other than Wee on the weapon. The prosecution nol prosses the charges.
The normal effect of nolle prosequi is to leave matters as if charges had never been filed. It's not an acquittal, which (through the principle of double jeopardy) prevents further proceedings against the defendant for the conduct in question. Rather, at least when it occurs before trial, nolle prosequi typically leaves the decision of whether to re-prosecute in the hands of the government. If the prosecution decides to bring charges again—for example, after it's gathered more evidence—it must file a new charging document. (People v. Daniels, 187 Ill. 2d 301 (1999), Kenyon v. Com., 37 Va. App. 668 (2002).)
However, dismissals are sometimes "with prejudice." A dismissal with prejudice means the prosecution can't ever re-file charges; a dismissal without it means the opposite.