A witness is one who sees and hears, understands, remembers, and is prepared to tell others. God said to Israel:

"This day I call heaven and earth as witnesses against you that I have set before you life and death, blessing and curses. Now choose life…." (Deuteronomy 30:19).

At least two witnesses were required to make a criminal charge against a man (see Deuteronomy 17:6). Sometimes gifts or tokens were exchanged (see Genesis 21:30) or memorial stones set up (see Genesis 31:52) as a witness to an agreement between two people, or between God and His People (see Exodus 24:1-8). God has made the nation of Israel His involuntary witness to the world that He exists and is their God, and has given them everything He promised them, good and bad. They are an ongoing witness to Him that because He is faithful to His promises, they have survived persecution for twenty centuries, and at the end of the age are being brought back to their God, even though they do not understand that God is doing this for them.

"'You are My Witnesses,' declares the Lord, 'and My Servant whom I have chosen, so that you may know and believe Me and understand that I am He. Before Me no god was formed, nor will there be one after Me. I, even I, am the Lord, and apart from Me there is no savior. I have revealed and saved and proclaimed-I, and not some foreign god among you. You are My witnesses,' declares the Lord, 'that I am God.'" (Isaiah 43:10-12).

When Jesus was on trial before Pilate the Jews brought false witnesses to testify against him, but they could not find two whose witness agreed, as required by the Law (see Mark 14:53-59).
After his resurrection Jesus made the apostles his witnesses: "…you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth." (Acts 1:8).

The apostle Paul was told: "You will be His witness to all men of what you have seen and heard." (Acts 22:15).

"And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come." (Matthew 24:14, KJV).

Note: The original Greek word is Martur-see also MARTYR.