## 1 Introduction
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Institutions/Scholars** & **Time** & **Definition** \\ \hline NASA [11] & 2011 & A highly integrated multi-physical field, multi-scale, and multi-probability simulation model for aircraft or systems that can reflect the function, real-time status, and evolution trend of the model to entities by using physical models, sensor data, and historical data. \\ \hline \multirow{2}{*}{[PERSON], Stargel D [8]} & \multirow{2}{*}{2012} & Digital twin is a complex system that integrates multi-physics, multi-scale, and multi-probability simulations. It uses the best physical model, sensor updates, and aircraft history to show the life of its aircraft digital twin. \\ \hline \multirow{2}{*}{Siemens [12]} & \multirow{2}{*}{2015} & Digital twin is an integrated system that creates virtual models for physical objects digitally, simulates their behavioral characteristics in reality, and applies data, models, and analysis tools to the entire product life cycle. \\ \hline \multirow{2}{*}{[PERSON], [PERSON] [6]} & \multirow{2}{*}{2017} & Digital twin is a virtual information structure that comprehensively describes potential or actual manufactured products from the microscopic atomic level to macroscopic geometry. \\ \hline \multirow{2}{*}{[PERSON] et al. [13].} & \multirow{2}{*}{2017} & Product digital twin refers to the total factor reconstruction and digital mapping of the working state and work progress of physical entities in the information space. It is a simulation model that integrates multi-physics, multi-scale, super-photorealistic and dynamic probability. \\ \hline \multirow{2}{*}{[PERSON] et al. [14].} & \multirow{2}{*}{2018} & Digital twin is a technology that establishes a dynamic virtual model of physical entities with multi-dimensional, multi-spatial scale, multi-disciplinary, and multi-physical quantities in a digital way, and simulates and depicts the attributes, behaviors, and rules of physical entities in the real environment. \\ \hline \multirow{2}{*}{[PERSON], [PERSON] [15]} & \multirow{2}{*}{2018} & Digital twin is a comprehensive digital representation of a single product, which simulates its actual behavior in the real environment through models and data (including the attributes, conditions, and behaviors of actual life objects). \\ \hline \multirow{2}{*}{[PERSON] et al. [16].} & \multirow{2}{*}{2019} & Digital twin is a technology that makes full use of data such as physical model, sensor update, and operation history, integrates multi-disciplinary, multi-physical, multi-scale, and multi-probability simulation processes, and completes mapping in virtual space to reflect the corresponding life cycle process of physical equipment. \\ \hline \multirow{2}{*}{[PERSON] et al. [17].} & \multirow{2}{*}{2020} & Digital twin is a technology that adds and expands new capabilities for physical entities through virtual and real interaction feedback, data fusion analysis, decision iteration optimization, and other means. \\ \hline \multirow{8}{*}{[PERSON], [PERSON] [18]} & \multirow{8}{*}{2021} & Digital twin is a complex system composed of five dimensions: physical entity, virtual model, twin data, service, and connection. It realizes monitoring, simulation, evaluation, prediction, optimization, control, and other functional services and virtual-real symbiotic interaction mechanism through a multi-dimensional virtual model, twin fusion data, and virtual-real closed-loop interaction mechanism. To monitor the changes in the physical world, simulate the behavior of the physical world, evaluate the state of the physical world, predict the future trend of the physical world, optimize the performance of the physical world, and control the operation of the physical world in the engineering applications at the unit level, system level, and complex system level. \\ \hline \multirow{8}{*}{[PERSON] et al. [19].} & \multirow{8}{*}{2022} & Digital twin can be understood as a concept: on the one hand, a digital space model (digital twin) reflecting the objective change law of physical objects is constructed in the digital space, and the model is used to deduce and predict the changing trend of physical objects, verify the control law, and thus change the evolution trajectory of physical objects. On the other hand, the observation of physical objects is continuously used to accumulate the cognition of physical objects to continuously improve its digital space model so that it can more accurately reflect the laws of physical change. \\ \hline \hline \end{tabular}
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