  Despite the growing popularity of digital twin (DT) developments, there is a
lack of common understanding and definition for important concepts of DT. It is
needed to address this gap by building a shared understanding of DT before it
becomes an obstacle for future work. With this challenge in view, the objective
of our study is to assess the existing DT from various domains on a common
basis and to unify the knowledge and understanding of DT developers and
stakeholders before practice. To achieve this goal, we conducted a systematic
literature review and analyzed 25 selected papers to identify and discuss the
characteristics of existing DT's. The review shows an inconsistency and
case-specific choices of dimensions in assessing DT. Therefore, this article
proposes a four-dimensional evaluation framework to assess the maturity of
digital twins across different domains, focusing on the characteristics of
digital models. The four identified dimensions in this model are Capability,
Cooperability, Coverage, and Lifecycle. Additionally, a weight mechanism is
implemented inside the model to adapt the importance of each dimension for
different application requirements. Several case studies are devised to
validate the proposed model in general, industrial and scientific cases.
