What physical state is achieved when outward radiation pressure perfectly balances inward gravitational collapse in a star?

Hydrostatic equilibrium.

Hydrostatic equilibrium represents the defining characteristic of a stable, main-sequence star, exemplified by the Sun in its current phase. This state is a precise dynamic balance between two immense, opposing forces acting upon the stellar mass. The primary inward force is gravity, constantly attempting to crush the star inward due to its vast mass. Counteracting this is the outward pressure generated by the energy output—stellar radiation pressure—originating from the nuclear fusion occurring in the core. When these two forces—gravity pulling in and radiation pushing out—are exactly matched, the star maintains a constant physical size and structure over vast timescales.