'Familiar spirit' definitions:

Definition of 'familiar spirit'

noun

A spirit (usually in animal form) that acts as an assistant to a witch or wizard [syn: familiar, familiar spirit]

From: GCIDE

Familiar \Fa*mil`iar\, a. [OE. familer, familier, F. familier, fr. L. familiaris, fr. familia family. See Family.]
1. Of or pertaining to a family; domestic. "Familiar feuds." --Byron.
Syn: familial. [1913 Webster]
2. Closely acquainted or intimate, as a friend or companion; well versed in, as any subject of study; as, familiar with the Scriptures. [1913 Webster]
3. Characterized by, or exhibiting, the manner of an intimate friend; not formal; unconstrained; easy; accessible. "In loose, familiar strains." --Addison. [1913 Webster]
Be thou familiar, but by no means vulgar. --Shak. [1913 Webster]
4. Well known; well understood; common; frequent; as, a familiar illustration. [1913 Webster]
That war, or peace, or both at once, may be As things acquainted and familiar to us. --Shak. [1913 Webster]
There is nothing more familiar than this. --Locke. [1913 Webster]
5. Improperly acquainted; wrongly intimate. --Camden. [1913 Webster]
Familiar spirit, a demon or evil spirit supposed to attend at call. --1 Sam. xxviii. 3, 7-9. [1913 Webster]

Familiar spirit 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.