arcane

arcane \ar-KAYN\, adjective:

Understood or known by only a few.

There are other arcane traditions that seem like superstitions to us, or, perhaps, are simply lost in translation. Some cyclists, for instance, believe that riders should shower instead of bathe because in some way water weight from baths is absorbed.
-- Allen Barra, "Tour de Lance", Wall Street Journal, July 28, 2009

While addressing a problem in the arcane field of mathematical logic, he imagined a machine that could mimic human reasoning.
-- Paul Gray, "Alan Turing", Time Pacific, March 29, 1999

Practitioners of this arcane art combine highly abstract mathematical deduction with some of the basic behavioral assumptions of micro-economics to produce theories of the behavior of voters, of representative assemblies, of bureaucracies, and even of courts.
-- Jerry L. Mashaw, Greed, Chaos, and Governance

Arcane comes from Latin arcanus, "shut, closed, secret," from arca, "chest, box."