If you mean something that's determined in court, it would be a finding of fact, or just finding, as opposed to a conclusion of law or a ruling. Findings often involve more than finding of fact - a jury verdict is a finding, even where it's just guilty or not guilty. However, they will always be based on questions of fact, to which the law may have been applied. A final decision may include findings and rationales to come to a final conclusion, applying the law to the facts, in a case where one or more judges are acting as finders of fact as well as judges of law.
If it's something that two parties to a case have agreed as a fact, which the court will then presume to be correct as the parties are agreed, that's a stipulation.