Leading Edge of the Wave of New Esoteric Understanding
Esotericism or, more neatly, Esoterism, the holding of esoteric opinions, derives from the Greek σωτερικός (es˘terikos), a compound of σω (es˘): "within", thus "pertaining to the more inward", mystic. Its antonym is exoteric.
Esoteric knowledge is that which is available only to a narrow circle of "enlightened", "initiated", or specially educated people. Esoteric items may be known as esoterica. In contrast, exoteric knowledge is knowledge that is well-known or public; or perceived as informally canonic in society at large.
In Western, English-speaking societies today, the term, "esotericism," is not necessarily used in the sense of mystical knowledge or practice, but has come informally to mean any perception or knowledge that is difficult to understand or remember, such as theoretical physics, or else that which pertains to the minutiae of a particular discipline, such as "esoteric" baseball statistics.