wit·ness 
(wĭtnĭs)
(wĭtnĭs)n.
1. One who can give a firsthand account of something seen, heard, or experienced: a witness to the accident.
2. Law
a. One who is called on to testify before a court.
b. One who is called on to be present at a transaction in order to attest to what takes place.
c. One who signs one's name to a document for the purpose of attesting to its authenticity.
3.
a. Attestation to a fact, statement, or event; testimony: She bears witness to these events in her letters.
b. Something that serves as evidence; a sign: The parody of the poem is a witness to the poem's popularity.
4.
a. One who publicly affirms religious faith.
b. Witness A member of the Jehovah's Witnesses.
v. wit·nessed, wit·ness·ing, wit·ness·es
v.tr.
1. To see or know by personal experience: witness a robbery; witness the birth of a new nation.
2.
a. To provide or serve as evidence of: The child's laughter witnessed her delight.
b. To consider as an example. Often used in the imperative: Even a widespread species can go extinct. Witness the passenger pigeon.
3. To testify to; bear witness of: The diary witnesses the difficult living conditions of the time.
4. To be the setting or site of: This old auditorium has witnessed many ceremonies.
5. To attest to the legality or authenticity of (a document) by signing one's name.
v.intr.
1. To furnish or serve as evidence: The fine buildings witness to the town's prosperity.
2. To testify to one's religious beliefs: "As they witnessed to their faith they brought others to believe" (Leon Morris).
[Middle English, from Old English, from wit, knowledge; see WIT1.]
witness·er n.