1) noun a person who testifies under oath in a trial (or a deposition which may be used in a trial if the witness is not available) with first-hand or expert evidence useful in a lawsuit. A party to the lawsuit (plaintiff or defendant) may be a witness. 2) noun a person who sees an event. 3) noun a person who observes the signing of a document like a will or a contract and signs as a witness on the document attesting that the document was signed in the presence of the witness. 4) verb to sign a document verifying that he/she observed the execution of the document such as a will.