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A witness is a person who testifies under oath in a trial or a deposition in the lawsuit. The plaintiff or defendant may be a witness. Witnesses have first-hand knowledge about matters relevant to the case and their testimony is subject to the applicable rules of evidence. A lay witness is an ordinary person who testifies based upon their personal knowledge and life experiences. A lay witness is distinguished from an expert testimony, who testifies based upon their qualifications of expertise in their field.
Many jurisdictions, as well as the Federal Rules of Evidence, now allow for lay witness opinion testimony in certain circumstances. Generally, all that need be shown to have a lay witness express an opinion is that the opinion is (a) rationally based on the perception of the witness and (b) helpful to either a clear understanding of his testimony or the determination of a fact which is in issue. A lay witness' opinion must not be based on knowledge outside the understanding of the ordinary person.