witness

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who can give a firsthand account of something seen, heard, or experienced.
• noun One who is called on to testify before a court.
• noun One who is called on to be present at a transaction in order to attest to what takes place.
• noun One who signs one's name to a document for the purpose of attesting to its authenticity.
• noun Attestation to a fact, statement, or event; testimony.
• noun Something that serves as evidence; a sign.
• noun One who publicly affirms religious faith.
• noun A member of the Jehovah's Witnesses.
• intransitive verb To see or know by personal experience.
• intransitive verb To provide or serve as evidence of.
• intransitive verb To consider as an example. Often used in the imperative.
• intransitive verb To testify to; bear witness of.
• intransitive verb To be the setting or site of.
• intransitive verb To attest to the legality or authenticity of (a document) by signing one's name.
• intransitive verb To furnish or serve as evidence.
• intransitive verb To testify to one's religious beliefs.

from The Century Dictionary.

• noun In geology, an eroded fragment of former more extensive strata, remaining in testimony of that which has departed.
• To bear witness or testimony; give evidence; testify.
• To take witness or notice.
• To give testimony to; testify; bear witness of, or serve as evidence of; attest; prove; show.
• [Witness in this sense is often used in the subjunctive imperatively or optatively, in many cases with inversion.
• To show by one's behavior; betray as a sentiment.
• To see or know by personal presence; be a witness of; observe.
• To see the execution of and affix one's name to (a contract, will, or other document) for the purpose of establishing its identity: as, to witness a bond or a deed.
• To foretell; presage; foretoken.
• =Syn.3. Perceive, Observe, etc. See see.
• noun Testimony; attestation of a fact or event; evidence: often with bear: as, to bear witness.
• noun One who or that which bears testimony or furnishes evidence or proof.
• noun One who is personally present and sees some act or occurrence, or hears something spoken, and can therefore bear witness to it; a spectator.
• noun A sponsor, as at a baptism or christening.
• noun In law:
• noun One who gives testimony on the trial of a cause; one who appears before a court, judge, or other officer, and is examined under oath or affirmation.
• noun One whose testimony is offered, or desired and expected.
• noun One in whose presence or under whose observation a fact occurred.
• noun One who upon request by or on behalf of a party subscribes his name to an instrument to attest the genuineness of its execution: more exactly, an attesting witness or a subscribing witness.
• noun In bookbinding, an occasional rough edge on the leaf of a bound book, which is a testimony that the leaves have not been unduly trimmed.

from the GNU version of the Collaborative International Dictionary of English.

• noun Attestation of a fact or an event; testimony.
• noun That which furnishes evidence or proof.
• noun One who is cognizant; a person who beholds, or otherwise has personal knowledge of, anything.
• noun One who testifies in a cause, or gives evidence before a judicial tribunal.
• noun One who sees the execution of an instrument, and subscribes it for the purpose of confirming its authenticity by his testimony; one who witnesses a will, a deed, a marriage, or the like.
• noun (Law) See under Privileged.
• noun [Colloq.] effectually; to a great degree; with great force, so as to leave some mark as a testimony.
• intransitive verb To bear testimony; to give evidence; to testify.
• transitive verb To see or know by personal presence; to have direct cognizance of.