Testimony

In law and in religion, testimony is a solemn attestation as to the truth of a matter.

The words "testimony" and "testify" both have a roots in the Latin testis, referring to the notion of a third person, disinterested witness.

In the law, testimony is a form of evidence that is obtained from a witness who makes a solemn statement or declaration of fact. Testimony may be oral or written, and it is usually made by oath or affirmation under penalty of perjury. To be admissible in court and for maximum reliability and validity, written testimony is usually witnessed by one or more persons who swear or affirm its authenticity also under penalty of perjury. Unless a witness is testifying as an expert witness, testimony in the form of opinions or inferences is generally limited to those opinions or inferences that are rationally based on the perceptions of the witness and are helpful to a clear understanding of the witness' testimony.

Legitimate expert witnesses with a genuine understanding of legal process and the inherent dangers of false or misleading testimony refrain from making statements of fact. They also recognize that they are in fact not witnesses to a alleged crime or other event in any way, shape or form. Their expertise is in the examination of evidence or relevant facts in the case. They should make no firm judgement or claim or accusation about any aspect of the case outside their narrow range of expertise. They also should not allege any fact they can't immediately and credibly prove scientifically.

For example, a hair sample from a crime scene entered as evidence by the prosecution should be described by an expert witness as "consistent with" a sample collected from the defendant, rather than being described as a "match". A wide range of factors make it physically impossible to prove for certain that two hair or tissue samples came from a common source.

Having not actually witnessed the defendant at the scene, the expert witness can not state for a fact that the sample is a match to the defendant, particularly when the samples were collected at different times and different palaces by different collectors using different collection methods. Ultimately, the testimony of expert witnesses is regarded as supportive of evidence rather than evidence in and of itself, and a good defense attorney will point out that the expert witness is not in fact a witness to anything, but rather an observer.

...
Wikipedia