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.

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."
Cite this page: N., Pam M.S., "FAMILIAR," in PsychologyDictionary.org, May 11, 2013, https://psychologydictionary.org/familiar/ (accessed August 5, 2021).