Prophet
A prophet is generally known as a mouthpiece of God. In other words, they are an intermediary between God and man. Old Testament prophets spoke for God, in the name of God, and carried God's message to others. The authenticity of a prophet is judged by their fruits as Jesus said that one should judge a prophet by his fruits (Matthew 7), and by checking whether his predictions come true.
Scripture
Deuteronomy 13:1-5
"If a prophet, or one who foretells by dreams, appears among you
and announces to you a miraculous sign or wonder, and if the sign
or wonder of which he has spoken takes place, and he says, 'Let us
follow other gods' (gods you have not known) 'and let us worship
them,' you must not listen to the words of that prophet or dreamer.
The LORD your God is testing you to find out whether you love him
with all your heart and with all your soul. It is the LORD your God
you must follow, and him you must revere. Keep his commands and
obey him; serve him and hold fast to him. That prophet or dreamer
must be put to death, because he preached rebellion against the
LORD your God, who brought you out of Egypt and redeemed you from
the land of slavery; he has tried to turn you from the way the LORD
your God commanded you to follow. You must purge the evil from
among you."
Deuteronomy 18:21-22
"You may say to yourselves, 'How can we know when a message has not
been spoken by the LORDÂ ?' If what a prophet proclaims in the name
of the LORD does not take place or come true, that is a message the
LORD has not spoken. That prophet has spoken presumptuously. Do not
be afraid of him."