I think of the word esoteric as meaning "the hidden part" or "the ignored part".

It is the hidden part for those who are too preoccupied with the mundane world to see it; the ones who know that the sky is blue but never think to ask why?

It is the ignored part for those who might know something to be true but are only willing to acknowledge what they can prove scientifically, then quietly sweep everything else under the rug of esoterica.

Esoterica, therefore, is anything that we choose to keep a secret from ourselves.