Esoteric

Adjective:  intended for or likely to be understood by a small number of people with a specialized knowledge or interest.

There are a few things that everyone understands, many more things that most people understand, and again, a few things that only a few people understand.  For the last category the word esoteric exists.  It can be used of a word, an idea, a concept or even an entire branch of study.  While researching-and I use the term lightly here-the word esoteric I discovered a new word to me:  exoteric.  Exoteric is the opposite of esoteric, something meant to be understood by a wide audience.  So there you have it, two words to describe two different audiences.  Esoteric, from the Greek for inner, for a small audience and exoterice, from the Greek for outside, for a wide or general audience.