While the text of the Fourth Amendment does not provide an exact definition of probable cause, the term has been fleshed out by the Supreme Court of the United States in multiple cases over the years.
Probable cause exists wherever there is a reasonable basis for believing that 1) evidence of a crime can be found in a particular location or 2) a crime might have been committed. This determination of reasonableness, in turn, is made by examining the totality of the circumstances.