Witness Definition:

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

The legal definition refers to the court-supervised recital of that sensory experience, in writing (deposition) but preferably in person and under oath (testimony).

In Book 4 of Justinian's Institutes, at 279, published in about 533 AD, can be found this Latin maxim:

"Testis oculatus unus plus valet quam auriti decem (one eye-witness is worth more than ten ear-witnesses).

Without the evidence of witnesses and their contribution to the law and justice system as reporters of events, the justice system would not be sustainable. Witnesses reconstruct events and tell a court what happened. Without witnesses, barring video or circumstantial evidence, a court cannot determine the facts of a controversial event.

It is out of respect for the witness that during trials, in some jurisdictions, the court and lawyers wear black robes.

At  Testifying: How To Give Evidence is more information about witnesses and their roles in the justice system.