Something that exists or has happened; an irreducible element of real existence or occurrence. collateral fact. A fact that is not central to the main issue or controversy. evidentiary fact. A fact that tends to prove, or is a necessary prerequisite for the proof of, another fact. jurisdictional fact. Such a fact as must exist before a court will exercise jurisdiction over a matter; for example, in a diversity case in federal court, that the parties are citizens of different states and that t...