The concept of smart city is getting more and more relevant for both academics and policy makers. Despite this, there is still confusion about what a smart city is, as several similar terms are often used interchangeably. This paper aims at clarifying the meaning of the word “smart” in the context of cities through an approach based on an in-depth literature review of relevant studies as well as official documents of international institutions. It also identifies the main dimensions and elements characterizing a smart city. The different metrics of urban smartness are reviewed to show the need for a shared definition of what constitutes a smart city, which are its features, and how it performs in comparison to traditional cities. Furthermore, performance measures and initiatives in a few smart cities are identified.