W is for Witness

witness (noun):

1. a person who sees an event first-hand or provides evidence
2. someone who has personal knowledge of something, or someone
3. someone who is asked to be present at a particular event and sign their name in order to prove that things have been done correctly

Acts 1:8 (NIV)

"But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth."