Digital Twins (DTs) constitute a growing and promising trend recognised by academia and industry. They are virtual replicas of distinctive objects, processes, buildings, or humans. DTs are used to reason about their physical counterparts’ functionalities, interactions, behaviours, and overall to plan optimal actions that they can perform or be subjected to. Given their intrinsic complexity, no standard definition nor a unified solution is yet available for designing and developing DTs. Intending to shed light on such a complex topic, we analysed the literature and derived a list of twelve pivotal characteristics of DTs. Such characteristics will be used as requirements for defining a Digital Twin Modelling Notation that will enable reasoning about the design of DT solutions.