A supernatural entity that serves a magician, witch, or sorcerer, often taking animal form. Common in fantasy and occult contexts.
The witch keeps a black cat as her familiar.
The sorcerer summoned his familiar and gave it orders.
Compound of 使い (messenger, servant) and 魔 (demon, magic). The term likely arose in modern fantasy and occult literature to translate the Western concept of a 'familiar spirit'.