Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. Closely acquainted or intimate, as a friend or companion;
      well versed in, as any subject of study; as, familiar with
      the Scriptures.

   3. Characterized by, or exhibiting, the manner of an intimate
      friend; not formal; unconstrained; easy; accessible. ``In
      loose, familiar strains.'' --Addison.

            Be thou familiar, but by no means vulgar. --Shak.

   4. Well known; well understood; common; frequent; as, a
      familiar illustration.

            That war, or peace, or both at once, may be As
            things acquainted and familiar to us. --Shak.

            There is nothing more familiar than this. --Locke.

   5. Improperly acquainted; wrongly intimate. --Camden.

   {Familiar spirit}, a demon or evil spirit supposed to attend
      at call. --1 Sam. xxviii. 3, 7-9.

Source : WordNet®

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