The 'digital twin' is now a recognised core component of the Industry 4.0 journey, helping organisations understand their complex processes, resources and data to provide insight into their business and help optimise their operations. However, there is still some confusion over the title, with the term 'digital twins' sometimes meaning different things to different people. This often leads to a mysticism and uncertainty around the technology and can mean that organisations are divided by a common language. This paper explores the different roles, definitions and purposes of a digital twin in an attempt to demystify the technology. By exploring different case studies and applications can we once and for all define a digital twin?