Noun

1. A person who has special knowledge about the inner workings of a group, organization, or institution.
   • 1923, "‘Big Board’ Failures," Time, 2 Jul.,

     Heavy losses were sustained in Simms Petroleum, which took a greater toll from supposed "Wall Street insiders" than from the general public.

   • 2007, Jonathan Clayton, "Profile: Zuma charmed wives and a nation," Times of London (UK), 19 Dec.,

     He is also an astute ANC insider who spent ten years on Robben Island alongside Nelson Mandela and the other “grandees” of the movement.

2. A person who is within an enclosed space.
   • 1849, Herman Melville, Redburn: His First Voyage, ch. 33,

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