The word esoteric was first used in a 1660 history of philosophy, where it was spelled esoterick. It was taken from Ancient Greek esoterikos, which meant "belonging to an inner circle" (this was later applied to knowledge to give us our modern definition). The term was first applied to the teachings of Pythagoras, which were meant only for his followers. That's from esotero, a comparative adverb of eso, meaning "within". Finally, it derives from Proto-Indo-European ens, "in". The opposite of esoteric is the word exoteric, meaning "understood by the general public". That comes from Ancient Greek exoterikos, "belonging to an outer circle", and traces to PIE ex-, "out of". Both words have been increasing in usage lately, but esoteric is used more than ten times as much, making exoteric the esoteric one.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.