Jamieson, Fausset and Brown on Leviticus 19:31

31. Regard not them that have familiar spirits--The Hebrew word, rendered "familiar spirit," signifies the belly, and sometimes a leathern bottle, from its similarity to the belly. It was applied in the sense of this passage to ventriloquists, who pretended to have communication with the invisible world. The Hebrews were strictly forbidden to consult them as the vain but high pretensions of those impostors were derogatory to the honor of God and subversive of their covenant relations with Him as His people.

neither seek after wizards--fortunetellers, who pretended, as the Hebrew word indicates, to prognosticate by palmistry (or an inspection of the lines of the hand) the future fate of those who applied to them.