## 2 Digital Twin in Literature
### Digital Twin Description in Literature  
Since the first-ever definition published by NASA, different authors have described DT in their own terms and based on its application. The definitions in the literature have referred to DT as a virtual or _digital model_[27; 28; 29; 30; 31; 32; 33; 34; 35], _layout_[36], _counterpart_[7; 37; 38; 35], _doppelanger_[38], _clone_[39], _footprint_[40], _software analogue_[41], _representation_[42; 43; 44; 45; 46], _information constructs_[2; 47], or _simulation_[20; 48; 49; 50; 51] of its physical counterpart. The first few definitions of DT that came out described DT only in respect to the aerospace/aeronautics industry which then ventured into manufacturing, which is why the words such as '_aircraft_'[23; 25; 30], _'vehicle_'[20; 26; 48], or _'airframe_'[29; 25] were replaced by '_system_'[27; 21; 22; 34; 35; 36; 40; 51; 52; 53; 50; 52], _'machine_'[28; 53], '_product_'[22; 36; 37; 44; 45; 51], '_object_'[9; 36; 41; 42; 43], '_entity_'[2; 9], '_asset_'[31; 33; 39; 40], '_device_'[41], or '_process_'[9; 21; 33; 36; 45; 54; 52; 23]. Ever since DT found its application in industries beyond manufacturing, the definition has also moved from just being restricted to non-living entities or industrial products, and can now be used to describe even complex biological systems as humans and trees, among others [43; 55]. One thing that binds most definitions of DT other than being a virtual representation of a physical object is the bidirectional transfer or sharing of data between the physical counterpart and the digital one, including quantitative and qualitative data (related to material, manufacturing, process, etc. ), historical data, environmental data, and most importantly, real-time data. Using these data, DT can perform such tasks as:  
* In-depth analysis of physical twin;
* Design and validation of new or existing product/process;
* Simulate the health conditions of physical twin;  
Figure 2: Timeline of evolution of Digital Twin. * Increase safety and reliability of physical twin;
* Optimization of part, product, process, or production line;
* Track the status of physical twin throughout its lifetime;
* Predict the performance of physical twin;
* Real-time control over physical twin. Definitions of DT tend to overlook its longevity; however, some authors consider DT as a cradle-to-gravel model, meaning that DT can be used over the entire life cycle, from the time of inception of the product until its disposal [23; 33; 44; 50]. However, [PERSON] and [PERSON] [47], who conceptualize the idea of DT, have defined a type of DT that is created even before its physical twin exists (see Section 3.3). [PERSON] and [PERSON] [56] in their review found eleven papers in which DT prepuls the physical twin. [PERSON] and [PERSON] also suggest that DT technology can have information related to the safe decommissioning of the product during its disposal phase. In addition, after the product is disposed, DT of one generation can help in the design and production of the next generation [57]. It is clear from the literature that DT is different from computer models (CAD/CAE) and simulation. Even though many organizations use the term 'Digital Twin' synonymously to 3D model, a 3D model is only a part of DT [33]. DT uses data to reflect the real world at any given point of time and thus can be used for observing and understanding the performance of the system and for its predictive maintenance [58]. Computer models, just like DT, are also used for the generic understanding of a system or for making generalized predictions, but they are rarely used for accurately representing the status of a system in real time. A lack of real-time data makes these models or simulations static, which means that they do not change or cannot make new predictions unless new information is fed to them [59]. However, having real-time data is not enough for DT to operate--the data also need to be loaded automatically to DT and the flow from physical to digital should be bidirectional. [PERSON] et al.