The rapid advancement of technology in smart cities has culminated in the development of the Urban Digital Twin (UDT), a tool that addresses various urban challenges and has applications across multiple fields. However, the lack of a universally accepted definition for UDT has resulted in significant conceptual and practical gaps. This study systematically examines various definitions of digital twins, outlining their fundamental components: the physical realm, digital realm, data, and their interconnections. It emphasizes the absence of standardization across different domains, particularly highlighting the need for further exploration of the social digital twin. Additionally, the inconsistent recognition of the virtual-to-physical connection and insufficient attention to data management issues, such as quality, security, and ownership, are identified as critical shortcomings. This analysis proposes an integrated definition of a digital twin as a dynamic virtual model that replicates a physical system, including its environment, processes, interactions, and stakeholders. This model maintains real-time, bidirectional connectivity, ensuring continuous, high-fidelity data exchange. By focusing on data integrity, security, and ownership, the proposed definition addresses current challenges and offers a robust framework for future research and innovation in smart city development.