Esotericism or Esoterism is a term with two basic meanings. In the dictionary sense of the term, it signifies the holding of esoteric opinions, and derives from the Greek ἐσωτερικός(esôterikos), a compound of ἔσω (esô): "within", thus "pertaining to the more inward", mystic. Its antonym is exoteric. In the scholarly literature, the term designates a series of historically related religious currents including Gnosticism, Hermetism, magic, astrology, alchemy, Rosicrucianism, the Christian Theosophy of Jacob Böhme and his followers, Illuminism,Mesmerism, Swedenborgianism, Spiritualism, the theosophical currents associated with Helena Blavatsky and her followers. There are competing views regarding the common traits uniting these currents, not all of which involve "inwardness", mystery or secrecy as a crucial trait.
Esoteric knowledge, in the dictionary (non-scholarly) sense, is thus 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.
Finally, it can be noted that esotericism, beside its scholarly and dictionary definitions, can be used in a loose, popular sense: not in order to denote e.g. mystical knowledge or practice, but rather informally to mean any perception or knowledge that is for the advanced individual such as theoretical physics, or that pertains to the minutiae of a particular discipline, such as "esoteric" baseball statistics. http://en.wikipedia.org/wiki/Esoterica