In its broadest sense, a special needs search is a search done without a search warrant issued by a court of law or without probable cause.
Almost all invasive searches in the United States must be done with a search warrant or with probable cause, a condition of the Fourth Amendment to the United States Constitution. However, the Supreme Court has recognized the legitimacy of a class of "special needs" searches, or searches that can be undertaken with no suspicion of any kind and without court order. Such searches must meet a three-part test established by the court: the nature of the search makes it impractical to obtain a warrant, the search is being undertaken to obtain evidence, and, there is a compelling interest that outweighs the invasiveness of the search.
Examples of special needs searches include a probation officer searching the home of a prisoner released on probation to ensure compliance with probation conditions, drug testing of government staff employed in positions of public trust such as police officers or prison guards, body and bag checks done before accessing secure areas of airports, and school locker checks.