2. (countable) one who sees or has personal knowledge of something
As a witness to the event, I can confirm that he really said that.
3. (countable, law) someone called to give evidence in a court
The witness for the prosecution did not seem very credible.
4. (countable) one who is called upon to witness an event or action, such as a wedding or the signing of a document
The bridesmaid and best man at a wedding typically serve as the witnesses.