Probable cause is a standard used in justifying certain police actions. For example, police need to have probable cause to believe evidence of a crime exists in requesting a search warrant to be issued. It is more than mere suspicion but less than the amount of evidence required for conviction.
The Fourth Amendment of the U.S. Constitution has two clauses. The first states that people have a right to be protected from unreasonable searches and seizures, and the second states that no warrant shall issue except upon probable cause. Probable cause is the level of evidence held by a rational and objective observer necessary to justify logically accusing a specific suspect of a particular crime based upon reliable objective facts.
For example, a police officer may claim there is probable cause for attempted theft when someone is found trespassing on private property late at night wearing a stocking mask, in order to justify stopping and searching the person for possession of criminal tools.