Legal Definition of Witness: (a) An individual who testifies under oath at a trial, a hearing, or before a legislative body. (b) To see or hear something take place. (c) To be present, and often to sign, a legal document, such as a will or deed. Having a witness sign lends authenticity to a document | Need to understand the legal definition of a witness? Learn how witnesses provide crucial testimony and observations, their roles in criminal trials and civil litigation, and why having a witness is crucial for establishing credibility, verifying facts, and preventing fraud. Click here to gain a clear understanding of the legal definition of a witness and its significance in various situations