Real Dictionary

witness
countable noun: A witness is someone who appears in a court of law to say what they know about a crime or other event.
countable noun: A witness is someone who writes their name on a document that you have signed, to confirm that it really is your signature.
countable noun: A witness to an event such as an accident or crime is a person who saw it.
verb: If someone witnesses your signature on a document, they write their name after it, to confirm that it really is your signature.
verb: If you say that a place, period of time, or person witnessed a particular event or change, you mean that it happened in that place, during that period of time, or while that person was alive.
verb: If you witness something, you see it happen.

witness in American English
an attesting of a fact, statement, etc.; evidence; testimony
to testify to
intransitive verb: to bear witness; testify; give or afford evidence
noun: an individual who, being present, personally sees or perceives a thing; a beholder, spectator, or eyewitness
transitive verb: to see, hear, or know by personal presence and perception

witness in British English
noun: a person who has seen or can give first-hand evidence of some event