What actually IS a familiar, speaking from a Lore point of view? I'm not asking what the use of a familiar is, or what kind of creatures can be a familiar- But in essence, what truly IS a familiar?
Is it an actual animal (Or other creature depending on the feats a character might have) that has, through the ritual to obtain your familiar, a magical bond with a particular spellcaster?
Is it (as I've been led to believe to be the case in 5e) an otherworldly spirit, summoned by and bound to a caster in the form of a regular animal (Or indeed other creature)?
Is it an inherent part of the caster's own being, made manifest in the form of a physical creature?
Is this kept purposefully vague and open to interpretation, making potentially all answers correct to different casters? Or is there something else entirely that I hadn't even conceived of while asking this question?