In Panosophy, what is esoteric is the Cosmic Psyche, the 99% of the Universe that lies within, which is inaccessible to our five physical senses. Mapping the Cosmic Psyche to obtain knowledge about why we behave as we do is not really a secret exercise, known only to initiates.
Rather, such a profound esoteric exercise is a potential for all humans should we be moved to let go of everything that prevents us from revealing the innermost secrets of the Universe: what it is, how it is intelligently designed, and the origin and destiny of our species.
About 1655, ‘understood only by a select few; secret; profound’, from Latinized form of Greek esoterikos ‘belonging to an inner circle’, from esōterō ‘more within’, comparative adverb of esō ‘within’, from PIE base *en ‘in’.