From Greek Ψυχή psyche, psykhē "breath, spirit, soul" and -logy, from λόγος Logos, the "principle governing the cosmos," the Word, God.
True psychology is "the knowledge of the relationship of the soul with God."
Esoteric psychology is the highest and most potent level of that knowledge, which is "private" and "interior" and is entered into in levels, according to the internal, initiatic process of the seeker.