A Fixed Point is a Term whose evaluation yields itself — self-Application finding stable configuration.
When a Function is applied to itself, the result may be a Term that, when evaluated further, produces itself again — a stable configuration under Reduction. This is a Fixed Point.
Fixed Points are central to the derivation because the entire relational structure is built on self-reference: Relating relates to itself, Closure maintains itself, Reflexive Form folds itself in. Fixed Point is the syntactic formalization of this pattern. Where earlier in the derivation self-reference was structural (a relational unit engaging its own boundary), here it becomes computable (a term evaluating to itself).
The term stratum distinguishes least and greatest fixed points — two complementary ways of finding stable configurations. The least fixed point builds up from below (the smallest self-consistent structure); the greatest fixed point settles down from above (the largest self-consistent structure). This duality echoes the relationship between Flow (directed forward) and Nucleus (consolidating down) that appears later in the derivation.