Concise Bible Dictionary:
Such are classed with false prophets. Possibly an hypnotic state which evil spirits could use, the object being to turn the people from the Lord to idolatry. If what they prophesied came true, they must not on that account be listened to: it was a test allowed of God to prove whether Israel loved the Lord (Deut. 13:1-5). We have the same caution in the New Testament as to the gospel and the confession of Jesus Christ come in flesh (Gal. 1:8-9; 1 John 4:1). In Jeremiah 27:9 the “dreamers” are classed with “diviners,” and in Jude 8 they are those that defile the flesh.