What Is Mass?

This paper reinterprets mass not as a primary attribute attached to an object, but as a reactive response exhibited when a pregeometric relational structure undergoes a change of motion while maintaining a stationary closure. The electron is modeled as a coupled system composed of a two-clock C₂ component and a three-clock C₃ component, whose complete state recurs after six clocks, forming a self-resonant closure. For a free electron that is already fully stationary, no permanent capacity for additional absorption is assumed. An externally supplied relational inconsistency temporarily deforms the closure and subsequently appears as restoration of the closure, a change of motion, or conversion into an open propagation mode. By describing this response through a structural correspondence with an LC resonant system, the restoring property of the closure is associated with a capacitive degree of freedom, while resistance to a change of motion is associated with an inductive reactance. The inertial mass is then interpreted as the physical-space representation of the latter coefficient. Furthermore, by expanding the one-cycle action difference between stationary and moving closures as an even function of the velocity ratio, the low-velocity relation between mass and rest energy is reduced to a dimensionless quadratic coefficient. This paper does not claim a complete derivation of the absolute mass or of E = mc². The central remaining task is to derive the quadratic coefficient independently from the rhombohedral geometry of the moving six-clock closure