From BiblePortal Wikipedia

Holman Bible Dictionary [1]

 Leviticus 20:22 Deuteronomy 18:11 Leviticus 20:27 Leviticus 20:6 1 Samuel 28:3 1 Samuel 28:8-19

The Hebrew word translated medium ( ob ) may refer to the spirit of a dead person, to the medium possessed by the spirit, or to images used to conjure up spirits. Manasseh made such images ( 2 Kings 21:6;  2 Chronicles 33:6 ). Josiah destroyed them as part of his reforms ( 2 Kings 23:24 ). Saul's success in quickly locating a medium ( 1 Samuel 28:8 ) points both to the popularity of the practice of consulting the dead and the difficulty of eradicating it.

 Isaiah 8:19 suggests a possible connection between the consulting of mediums and ancestor worship. Those to be consulted are termed “fathers” and “gods.” (Compare   1 Samuel 28:13 where Samuel is described as elohim or “god.”) The chirping and muttering of the spirits perhaps refers to the inarticulate sounds which must be interpreted by the medium. Consulting of mediums defiled the land and was described as prostitution. God's people were to trust God in times of distress and not resort to other “gods” in an attempt to learn the future.

Webster's Dictionary [2]

(1): ( n.) The mean or middle term of a syllogism; that by which the extremes are brought into connection.

(2): ( n.) A substance through which an effect is transmitted from one thing to another; as, air is the common medium of sound. Hence: The condition upon which any event or action occurs; necessary means of motion or action; that through or by which anything is accomplished, conveyed, or carried on; specifically, in animal magnetism, spiritualism, etc., a person through whom the action of another being is said to be manifested and transmitted.

(3): ( n.) See Mean.

(4): ( n.) That which lies in the middle, or between other things; intervening body or quantity. Hence, specifically: (a) Middle place or degree; mean.

(5): ( n.) The liquid vehicle with which dry colors are ground and prepared for application.

(6): ( n.) A trade name for printing and writing paper of certain sizes. See Paper.

(7): ( a.) Having a middle position or degree; mean; intermediate; medial; as, a horse of medium size; a decoction of medium strength.

(8): ( n.) An average.

Baker's Evangelical Dictionary of Biblical Theology [3]

See Divination; Idolatry Idol; Necromancy

The Nuttall Encyclopedia [4]

In modern spiritualism a person susceptible to communication with the spirit-world.