A plenitude of digital twin reviews have been published in recent years, most of which, however, solely review project-specific digital twin implementations or focus on specific application areas within civil engineering. Despite the widespread adoption of digital twin applications in recent years in civil engineering, there is a lack of agreement on a common definition of digital twins. A common definition and, furthermore, an understanding of digital twin architectures and the composition of the internal elements would advance digital twin implementations, enhancing the reliability and performance of digital twins. In this study, a multivocal review of digital twins in civil engineering is conducted, aiming to provide a scientific basis for reaching a consensus on the abstract architecture and the composition of the internal elements of digital twins. Methodologically, the review is conducted in a twofold manner, accounting for both peer-reviewed, indexed literature, as well as for material that may report on research and development projects and industrial applications not indexed in well-accepted citation databases. The main contribution of this study is a systematic overview of existing architectures of digital twins implemented in civil engineering, including the internal elements. Furthermore, a digital twin definition and a generic reference architecture are proposed. It is expected that the definition and the reference architecture will serve as blueprints for digital twin implementations in civil engineering, providing a common understanding and enhancing the reliability and performance of digital twins.