This paper aims to define Cloud Computing by analyzing over 20 existing definitions and extracting a consensus and a minimum common denominator. The authors discuss the concept of Cloud Computing, emphasizing its shift in providing computing infrastructure to the network to reduce management costs. They highlight the confusion between Cloud and Grid technologies, particularly in terms of resource sharing, heterogeneity, virtualization, security, high-level services, architecture, and payment models. The paper proposes a comprehensive definition of Cloud Computing, focusing on key features such as scalability, pay-per-use utility model, and virtualization. It also compares Clouds and Grids, noting their similarities and differences, and discusses the need for standardization and quality of service (QoS) enforcement. The authors conclude that while Clouds and Grids share some concepts, Clouds offer unique advantages like on-demand resource sharing, enhanced security, and user-friendliness. They emphasize the importance of standardization and QoS enforcement to build confidence in Cloud Computing.This paper aims to define Cloud Computing by analyzing over 20 existing definitions and extracting a consensus and a minimum common denominator. The authors discuss the concept of Cloud Computing, emphasizing its shift in providing computing infrastructure to the network to reduce management costs. They highlight the confusion between Cloud and Grid technologies, particularly in terms of resource sharing, heterogeneity, virtualization, security, high-level services, architecture, and payment models. The paper proposes a comprehensive definition of Cloud Computing, focusing on key features such as scalability, pay-per-use utility model, and virtualization. It also compares Clouds and Grids, noting their similarities and differences, and discusses the need for standardization and quality of service (QoS) enforcement. The authors conclude that while Clouds and Grids share some concepts, Clouds offer unique advantages like on-demand resource sharing, enhanced security, and user-friendliness. They emphasize the importance of standardization and QoS enforcement to build confidence in Cloud Computing.