A mistrial is a trial that ends without a verdict or judgment on the merits. In U.S. courts, that usually happens when a jury cannot agree (a hung jury), or when a serious procedural error or misconduct would make a fair trial impossible and the judge stops the case and orders a new trial.
Common grounds include improper admission of prejudicial evidence, misconduct by lawyers, parties, or jurors, and other defects that undermine fairness. A mistrial is not an acquittal or conviction; the case can often be retried with a new jury, though double-jeopardy rules can bar a second prosecution in some situations.
Source: Cornell Law School Legal Information Institute — Mistrial