The office of being a prophet in the old testament was something very serious, and as in all existing office, there are always false and true, and in the people of Israel there were many false prophets, so Elijah once resigned himself to think that he was the only prophet of God left on earth.
First of all, what is a prophet? Is a person who made predictions through the intervention of God himself. So what is a false prophet? This is a person who presumes that God revealed something to him, but in reality God never said anything to him. As simple as that. And in the book of Jeremiah we will find a fight against false prophets:
23 Am I a God at hand, saith the Lord, and not a God afar off?
24 Can any hide himself in secret places that I shall not see him? saith the Lord. Do not I fill heaven and earth? saith the Lord.
25 I have heard what the prophets said, that prophesy lies in my name, saying, I have dreamed, I have dreamed.
26 How long shall this be in the heart of the prophets that prophesy lies? yea, they are prophets of the deceit of their own heart;
Jeremiah 23:23-26 (KJV)