This work introduces the spaton as the minimal physical unit of the Spatum substrate,treated as a physical medium constituting space itself. The objective of this article is not topresent dynamical models, detailed equations, or phenomenological fits, but to establishontological definitions, regime structure, and operational relations that any subsequentdevelopment within the Spatum theory must obey.The spaton is defined as an indivisible unit of local space with a continuous volumetric statebounded by finite physical limits. Three volumetric regimes—maximum compression(Vmin), a dynamic coherence regime (Vdyn), and maximum expansion (Vmax)—organizethe stability of coherent propagation, the emergence of time as accumulated delay, and theoperational definition of the speed of light as a coherence transmission rate. The presenttext remains strictly foundational; applications and empirical programs are deferred tomodular future works.