in purely mythological terms, a familiar would be a pet or supernatural spirit that supposedly lives with another individual and acts as their servant. Witches were tried on the basis they possessed familiars.
What is FAMILIAR?
Written by Pam MS, NCSP | Fact checked by Psychology Dictionary staff
FAMILIAR: "The majority of accused (and some killed after being found guilty) witches in medieval times were found guilty on the basis of having a familiar."