from Greek. Eso means within; esotericos denotes inner. The
opposite terms are exoteric: ex meaning out; exo denoting outside
(e.g. the word exit). In contrast to exoteric knowledge which was
given to the masses, esoteric knowledge is intended for or
understood by only a small group, the inner circle. It is not publicly
disclosed, it is confidential.
An esoteric subject matter can be abstract, complex, complicated,
and ultimately incomprehensible. Interestingly enough, some of the
fathers of quantum physics of the 20th century called their discipline
also incomprehensible.
Esoteric also means recondite and achromatic. A recondite subject
is beyond the reach of average intelligence. Achromatic is derived
from the Greek word across, which indicates height, summit, outer
end, tip, topmost, and extreme.