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A witness is someone with firsthand knowledge of an event, or a person who sees a second person sign a document, then adds their own signature confirming (or attesting) that the first signature is genuine.

A witness may testify under oath during a trial or other legal proceedings, and they should have first hand evidence to offer, or expert evidence to provide during a legal proceeding. 

A witness could be called by the court or by any party’s request, according to Rule 614 of the Federal Rules of Evidence (FRE); a witness could be excluded under Rule 615 of the FRE. A non-expert witness is called a “lay witness”, and they may only offer an opinion if it is based on their perception, if it is helpful to understand or determine a fact in issue, or if it is not based on scientific, technical knowledge, according to Rule 701 of the FRE; for example, a witness could say the speed of a car is really fast, yet they could not say that the car is running 120 miles per hour.

See also: eyewitness, earwitness, expert witness

[Last updated in March of 2022 by the Wex Definitions Team]