What it means
A witness is a person who gives evidence in a proceeding, either orally on oath or affirmation, or in writing through an affidavit or witness statement. Witnesses include people who observed relevant events and expert witnesses who give opinion evidence within their field of expertise. After giving evidence-in-chief, a witness can be cross-examined by the other party. A witness can be compelled to attend by subpoena.
How it's used
The neighbour was called as a witness to describe the noise she heard on the night.