Familiar spirit - (Easton's Bible Dictionary)
Sorcerers or necormancers, who professed to call up the dead to answer questions, were said to have a "familiar spirit" (Deut. 18:11; 2 Kings 21:6; 2 Chr. 33:6; Lev. 19:31; 20:6; Isa. 8:19; 29:4). Such a person was called by the Hebrews an 'ob, which properly means a leathern bottle; for sorcerers were regarded as vessels containing the inspiring demon. This Hebrew word was equivalent to the pytho of the Greeks, and was used to denote both the person and the spirit which possessed him (Lev. 20:27; 1 Sam. 28:8; comp. Acts 16:16). The word "familiar" is from the Latin familiaris, meaning a "household servant," and was intended to express the idea that sorcerers had spirits as their servants ready to obey their commands.