Underlayn

Underlayn is a term used in some discussions of layered systems and speculative fiction to denote a hidden or base layer that underlies surface phenomena. It is a neologism; there is no single standard definition, and its meaning can vary by context.

In computing and information architecture, underlayn can refer to the persistent data layer and policy framework that coexists with, but is not exposed through, user interfaces. Characteristics include stability, governance, data integrity, versioning, and interoperability with front-end layers. It contrasts with an overlay that represents surface-level features or services.

In literature and world-building, the underlayn may describe the substrate reality or social fabric that shapes events—an unseen infrastructure of power, memory, or natural law. The term is sometimes used to discuss how invisible forces influence visible systems.