With the increasing integration of renewable energy resources into power grids, system inertia is decreasing considerably. This trend poses major challenges to transmission system operators and requires a comprehensive understanding of inertia-related information to formulate effective strategies that ensure power system frequency stability. In this study, an inertia security evaluation methodology is proposed from a frequency stability perspective, including its definition, quantitative assessment, and application. First, the definition and mathematical underpinnings of the inertia security region (ISR) are introduced. Then an evaluation method for the ISR is established, considering various frequency stability control strategies based on the size of the disturbance. Two quantitative indices, the inertia security ratio and inertia adjustable capability, are introduced to assist grid operators in determining the system inertia security status and assessing the inertia adjustability capability. Furthermore, for inertia deficits, a sensitivity-based inertia allocation approach is proposed for credible contingencies. Finally, the efficacy of the proposed ISR approach is validated using case studies on a modified 39-bus system and a practical large-scale power grid in China.