A new trial is a repeat of a legal trial, conducted because of a flaw or problem with the original. Courts only grant requests for new trials in very special circumstances, as they are expensive and there may be civil rights concerns about dragging out the legal process or retrying an accused person. In the United States, for example, a new trial after an acquittal would be considered double jeopardy under the Constitution, and would not be permissible.
There are three different reasons why a court may agree to conduct a new trial. The first is the lack of a verdict in the original trial as a result of a hung jury. Juries may hang, meaning that they are unable to reach agreement on a verdict, in a variety of circumstances. Judges typically encourage them to renegotiate and work with each other on an agreement, but they cannot compel juries to agree, and may decide to end the trial when it is clear that the jury will never reconcile its differences. A trial may also lack a verdict if something occurs to stop it partway through, such as a prejudicial event that would make it impossible for the trial to continue with the same jury.
Another reason for a new trial is an order from an appellate court. A convicted person has the right to appeal, and in the appeal hearing, she will present information to compel the court to overturn the original verdict. The court may agree to reverse the conviction and follow with an order to the original court to repeat the trial. The purpose of the new trial is to achieve a fair verdict, and the appellate court can issue a warning reminding the lower court about certain aspects of trial procedure, such as the need to word jury instructions with more care.
Finally, a trial may have a legal defect so profound that the verdict cannot be allowed to stand. A judge can review the facts of the case to decide if a defect was minor and the outcome would likely have been the same, or if it was significant enough to impede the course of justice. A defense attorney sleeping through a trial, for example, would be grounds for a new trial, as would the introduction of fabricated evidence to support a key point upon which a conviction hinges.
New trials are not desirable because of the expense and investment of labor, but the court will move forward with a new trial if it is necessary to make sure someone receives fair treatment in the justice system.