Hybrid applications (apps) have become one of the most attractive options for mobile app developers thanks to its support for portability and device-specific features. Android hybrid apps, for example, support portability via JavaScript, device-specific features via Android Java, and seamless interactions between them. However, their interoperation semantics is often under-documented and unintuitive, which makes hybrid apps vulnerable to errors. While recent research has addressed such vulnerabilities, none of them are based on any formal ground. In this paper, we present the first formal specification of Android interoperability to establish a firm ground for understanding and reasoning the interoperations. We identify its semantics via extensive testing and thorough inspection of Android source code. We extend an existing multi-language semantics to formally express the key features of hybrid mechanisms, dynamic and indistinguishable interoperability. Based on the extensions, we incrementally define a formal interoperation semantics and disclose its numerous unintuitive and inconsistent behaviors. Moreover, on top of the formal semantics, we devise a sound and efficient type system that can detect bugs due to the unintuitive inter-language communication. We show that it detects more bugs more efficiently than HybriDroid, the state-of-the-art analyzer of Android hybrid apps, in real-world Android hybrid apps.