The scientific discussion on the Digital Twin gained substantial momentum within the last couple of years. In the context of Industry 4.0, the Digital Twin in the broadest sense refers to the concept of a coupled virtual representation of a physical asset. The asset types may range from products and processes to whole production systems. Different authors offer a variety of interpretations on the actual nature of the Digital Twin, often defining it via implicit functional aspects rather than giving explicit definitions. At the same time, some authors use the term merely as a catchphrase, thus applying blur to this emerging paradigm. At this point, a comprehensive understanding as well as a unifying model of the Digital Twin are absent. This paper therefore provides a structured review of the body of relevant literature with the specific focus on finding explicit definitions of the term "Digital Twin" as well as stated models characterizing the Digital Twin concept. As a result, a relevance assessment provides the most important literature regarding the research questions. The found explicit definitions, being partly conflicting, are presented. Similar notions like Product Avatar and Digital Shadow are assorted accordingly. Further, depictions of Digital Twin models stated in the most relevant literature as well as derived Digital Twin purposes are provided. Thus, this paper extends the theoretical foundation, thereby setting a basis for future augmented Digital Twin modeling.