witness

v. look on, see, observe; testify, vouch for, corroborate; be an eye-witness; note, notice

n. testifier; onlooker, bystander; eye-witness; testimony

A witness is someone who has firsthand knowledge about a crime or dramatic event through their senses (e.g. seeing, hearing, smelling, touching) and can help certify important considerations to the crime or event. A witness who has seen the event firsthand is known as an eye-witness. Witnesses are often called before a court of law to testify in trials.
See more at Wikipedia.org...

(v. t.)
To see the execution of, as an instrument, and subscribe it for the purpose of establishing its authenticity; as, to witness a bond or a deed.
  

(v. t.)
To see or know by personal presence; to have direct cognizance of.
  

(v. t.)
To give testimony to; to testify to; to attest.
  

(v. i.)
To bear testimony; to give evidence; to testify.
  

(v. i.)
That which furnishes evidence or proof.
  

(v. i.)
One who testifies in a cause, or gives evidence before a judicial tribunal; as, the witness in court agreed in all essential facts.
  

(v. i.)
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.
  

(v. i.)
One who is cognizant; a person who beholds, or otherwise has personal knowledge of, anything; as, an eyewitness; an earwitness.
  

(v. i.)
Attestation of a fact or an event; testimony.
  

Person who witnesses a crime and gives evidence in judicial proceedings. The EU supports witnesses being able to give evidence in another Member State via video-conference technology. (see judicial-criminal)

The regular definition of this word is a person who perceives an event (by seeing, hearing, smelling or other sensory perception). - (read more on Witness)