This paper proposes a conceptual framework for geometrically characterizing the small-disturbance stable operating area of an autonomous microgrid. A concept of stability region of distributed generation (DGSR) is established on a general differential-algebraic-equation model of autonomous microgrids. It provides a graphical tool to characterize the limits for a microgrid to accommodate the distributed, volatile, and uncertain power generation of renewable energy, inside which the stability of the system can be guaranteed. The Kernel Ridge Regression method is employed to accurately and quickly estimate the boundary of the DGSR, leading to its approximate expression in an analytical form. The DGSR allows for a straightforward and intuitive evaluation of stability and provides guidance for adjustments to enhance it. Case studies on two autonomous microgrids are used to verify the performance of the proposed DGSR and its potential applications.