EchoKey: A Unified Mathematical Framework for Complex Systems introduces a multidimensional formalism for modeling, controlling, and interpreting the behavior of nonlinear and emergent systems. Synthesizing concepts from quantum mechanics, fractal geometry, recursion theory, synergy analysis, and outlier management, the framework defines a governing equation that integrates seven core principles: cyclicity, recursion, fractality, regression, synergy, refraction, and discrete perturbation. This unified structure enables cross-domain equation composition, allowing researchers to import and interconnect models from physics, biology, economics, and beyond. The document presents the theoretical foundations, governing operators, convergence proofs, and real-world simulations—demonstrating EchoKey’s versatility as a compositional language for adaptive systems. Designed for both mathematical rigor and practical implementation, EchoKey offers a scalable foundation for future research in systems theory, artificial intelligence, and complex adaptive modeling.