insider

noun2

1. •
   A person who has special knowledge about the inner workings of a group, organization, or institution.

   • Heavy losses were sustained in Simms Petroleum, which took a greater toll from supposed "Wall Street insiders" than from the general public. — 1923
2. •
   A person who is within an enclosed space.

   • To the insider, the ceiling is like a small firmament twinkling with astral radiations. — 1849