Probable cause is more than mere suspicion but less than the proof needed for conviction. It means a reasonable belief, based on facts and circumstances, that a crime has been committed and that the person or place is connected to it.
Probable cause is required for arrests, search warrants, and grand jury indictments. Challenging whether probable cause existed is one of the most common pretrial motions in criminal defense.
If a court finds an arrest or search lacked probable cause, the resulting evidence may be suppressed (excluded from trial), which often ends or substantially weakens the prosecution's case.