What is Trier Of Fact?

Legal Definition
A trier of fact, or finder of fact, is a person, or group of persons, who determines facts in a legal proceeding, usually a trial. To determine a fact is to decide, from the evidence, whether something existed or some event occurred. Various aspects of a case that are not in controversy may be the "facts of the case" and are determined by the agreement of the separate parties; the trier of fact need not decide such issues.
-- Wikipedia
Legal Definition
A judge or jury that determines questions of fact in a trial.

See Jury trial and Bench trial.