A Pillar is a Sacred Artifact that can retain new memories formed within a loop, even after the loop resets with the Great Flood of Time. They support the worlds of their origin. Pillars also refers to the wielders of such Sacred Artifacts.
In a more general sense, pillars refer to things that connect AETHER and NETHER together. Conceptually, this relates to connecting opposite concepts such as Yang and Yin, light and darkness, or sky and earth. The permanent Gates of Tokyo, pillars of light that penetrate the earth and the sky, are also pillars, according to Breke recalling the memories of Azathoth.
Relation to Representatives
Representatives are often but not always Pillar owners. Similarly, Pillar owners are not always the Representatives of their worlds. A Representative that is also a Pillar has a major advantage in the Game, having a clear objective to win for their World while being able to refer to the memories of past loops to give them an information advantage in future loops.