X is a new generation of multi-disciplinary and unified modeling language for complex systems, which is designed to provide a unified modeling and simulation language for the whole process (requirement, design, verification, etc.), multi-disciplinary (mechanical, electrical, hydraulic, control, etc.), multi-granularity (parts, components, devices, subsystems, systems, and even architectures), and multi-featured (continuity, discretization, hybridization, intelligence, etc.) of complex systems.

X language fully supports Model-Based Systems Engineering (MBSE), provides standardized graphical modeling descriptions in the product conceptual design stage, and also automatically compiles and converts the standardized graphical model into a textual underlying simulation model, which is driven by the simulation engine to support seamless and integrated simulation of the whole system, the whole process, and multiple perspectives, and realizes unified and integrated description and simulation models from the conceptual model design, the system architecture design, and the multiphysical domain model to the simulation model. The simulation engine is driven by the simulation engine and supports seamless integrated simulation of the whole system and multiple perspectives, realizing unified integrated description and integrated simulation from conceptual model design, system architecture design, multi-physical domain model to simulation model.

X language was born in the MIST laboratory of Beijing University of Aeronautics and Astronautics (BUAA). It is a modeling language with independent property rights developed by Mr. Zhang Lin's team in the national key research and development project "Complex Product Modeling and Simulation System".

X language is named X because the design concept of X language integrates the modeling capabilities of existing mainstream modeling languages (e.g., SysML, Modelica, DEVS, etc.), which can be interpreted as a cross-collection of the advantages of multiple modeling languages, and is therefore named X language.

X is an object-oriented modeling language, which is designed for the characteristics of multi-domain, multi-granularity and multi-features of complex systems, and four specific classes (continuous, discrete, intelligent and coupled) and three auxiliary classes (connector, record and function) can be used to realize the modeling and description of complex systems in a fast, accurate and efficient way. In addition, in order to better support MBSE's complex system modeling and simulation, the X language has been designed to provide both graphical and textual modeling formats for the above seven categories in a consistent way. In this way, modelers can use X to complete the whole process of complex system-oriented development.