“Probable cause” is the standard by which judges evaluate:
There is probable cause if the facts support an objective belief that the person to be arrested has committed a crime—or that the place or item to be searched bears evidence of a crime. (See When the Police Can Make an Arrest: Probable Cause.) The obvious question about probable cause is: How likely must it be that the defendant committed a crime, or that the place or item contains criminal evidence?
Courts are extremely reluctant to describe probable cause—and reasonable suspicion—in terms of numbers. But judicial opinions suggest that probable cause is something less than preponderance of the evidence, the typical standard for deciding civil cases. Preponderance of the evidence requires proof that a fact is more likely than not to be true. If probable cause is lower than the preponderance-of-evidence standard, then it doesn’t require an over-50% likelihood. In that way, the term “probable” is somewhat of a misnomer.
Ultimately, the best one can say about probable cause is that it doesn’t require an actual probability. (See Texas v. Brown, 460 U.S. 730 (1983), Illinois v. Gates, 462 U.S. 213 (1983).)