The second article in the Quantum π-Unification Series establishes a fully defined, operational, and mathematically rigorous formulation of the Quantum π for molecular systems. Unlike classical π-electron theory—which only describes delocalized electrons—the Quantum π introduced here represents a phase-information invariant governing chemical stability, resonance, symmetry, and reactivity. This work develops: 1. the conceptual foundations of the Quantum π, 2. its mathematical structure (phase operator, symmetry factor, information contribution), 3. the connection with chemical resonance, electronegativity flow, and energy minimization, 4. prediction rules for molecular stability and reactivity. The article also introduces the π-Stability Index (PSI) and the Quantum π-Symmetry Number, two new descriptors that unify chemical information, electronic delocalization, and energetic behavior.