A motion to quash is a formal request to have a prior order or ruling invalidated, nullified or voided. It is applicable in both civil and criminal matters. This type of request can only be made in certain circumstances and is usually, but not always, presented before the case goes to trial in a higher court.
To secure relevant information, attorneys in criminal and civil cases often issue orders compelling witnesses to appear at or present specific information for depositions and/or trials. While it is against the law to ignore this type of order, which is called a subpoena, filing a motion to have it quashed is permitted. This usually occurs if the subpoena isn't drafted properly, if the information being sought is legally immune from disclosure, or if providing the information would create an unreasonable hardship.
Conversely, a motion to quash a prior court ruling can only be made when there is reason to believe the court ruled erroneously.