a prophet: someone who speaks for God, delivering a message that comes from God rather than from themselves
A prophet is God's mouthpiece. The word does not mean someone who predicts the future, though predicting is sometimes part of it. At its root it names a person who speaks out a message on behalf of another, and in the Bible that other is God. The prophet's authority is borrowed: the message is not their own opinion but what God has given them to say.
The New Testament uses the word along a single line that runs through history. Most often it points back to the long line of prophets God raised up in Israel, whose written words became scripture, so that "the Prophets" can stand for a whole section of the Bible. It also names a recognized role in the early church, where certain believers spoke God-given messages to build up the congregation. And it is the title people reached for when they tried to place Jesus, or John before him, in the category of God's authorized spokesmen.
Because the heart of the word is "one who speaks for the divine," it can even be stretched to a pagan seer or sage, as when Paul quotes a Cretan poet and calls him "a prophet of their own." That stretch is the exception that proves the rule: the same core meaning is at work, only the god being spoken for has changed.