There is some consensus among researchers that the first urban civilization labeled a ‘city’ was Sumer in the period 3,500–3,000 BC. The meaning of the word, however, has evolved with the advancement of technology. Adjectives such as digital, intelligent, and smart have been prefixed to ‘city’, to reflect the evolution. In this study, we pose the question: What makes a ‘Smart City’, as opposed to a traditional one? We review and synthesize multiple scientific studies and definitions, and present a unified definition of Smart City—a complex concept. We present the definition as an ontology which encapsulates the combinatorial complexity of the concept. It systematically and systemically synthesizes, and looks beyond, the various paths by which theory and practice contribute to the development and understanding of a smart city. The definition can be used to articulate the components of a Smart City using structured natural English. It serves as a multi-disciplinary lens to study the topic drawing upon concepts from Urban Design, Information Technology, Public Policy, and the Social Sciences. It can be used to systematically map the state-of-the-research and the state-of-the-practice on Smart Cities, discover the gaps in each and between the two, and formulate a strategy to bridge the gaps.