The Fourth Amendment to the Bill of Rights of the United States of America gave rise to the term "probable cause". In essence the Fourth Amendment carries 2 clauses. The first clause 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.
It is widely acknowledged that the probable cause requirement is, in many ways, more important than the reasonableness clause. Not all searches and seizures require warrants. Examples of which are automobile searches and arrests in a public place. However, the US Supreme Court has interpreted warrantless searches and seizures as unreasonable unless preceded by probable cause. This means that as a general rule, most searches and seizures require probable cause.
Although there is no written definition of the term "probable cause", but generally speaking based on the established rulings from precedence, probable cause is:
(i) where known facts and circumstances, of a reasonably trustworthy nature, are sufficient to justify a man of reasonable caution or prudence in the belief that a crime has been or is being committed;
(ii) what would lead a person of reasonable caution to believe that something connected with a crime is on the premises of a person or on persons themselves; and/or
(iii) the sum total of layers of information and synthesis of what police have heard, know, or observe as trained officers.