Whether in newspaper articles, books, or conversations about Islam, the ‘Muslim world’ is a commonplace term. Yet it was only coined in the late nineteenth century, and didn’t gain wider currency till the 1920s. Moreover, the ‘Muslim world’ wasn’t even a Muslim invention. In this episode, we trace the history of this term which, over the course of a century, came to serve many different purposes when it was taken up by a range of political and religious figures, Muslim and non-Muslim alike. We begin by asking how Muslims thought about geography before this new term was invented, then we follow the changing geopolitical contexts in which this relatively recent label acquired the familiarity of apparent commonsense. Along the way, we travel from the late Ottoman Empire and the US Philippines to the Muslim World Congress and Muslim World League through which Pakistan and Saudi Arabia staked rival claims of leadership over the Muslims of the world. Nile Green talks to Cemil Aydin, author of The Idea of the Muslim World: A Global Intellectual History (Harvard University Press, 2017).