noun

• (uncountable) Attestation of a fact or event; testimony.
  "She can bear witness, since she was there at the time."
• (countable) One who sees or has personal knowledge of something.
  "As a witness to the event, I can confirm that he really said that."
• (countable, law) Someone called to give evidence in a court.
  "The witness for the prosecution did not seem very credible."
• (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."

verb

• (transitive) To furnish proof of, to show.
  "This certificate witnesses his presence on that day."
• (transitive) To take as evidence.
  "Depression often goes undetected until it is too late . Witness the recent White House suicide."
• (transitive) To see or gain knowledge of through experience.
  "He witnessed the accident."
• (intransitive, construed with to or for) To present personal religious testimony; to preach at (someone) or on behalf of.
  "Instead, Niebuhr's God was the God witnessed to in the Hebrew Scriptures and the New Testament, the Bible of the Christian world."