This book, the first to explore the politics of definitions from an interdisciplinary perspective, encourages readers to reconsider the value and limits of definitions in confronting antisemitism and Islamophobia. In recent years, definitions of antisemitism and Islamophobia have become central to the struggle to combat the hostility, harassment and discrimination experienced by Jews and Muslims. Yet these definitions have also provoked fierce controversy: critics have questioned whether they are fit for purpose, or have criticised them as unwelcome attempts to restrict freedom of expression. In this edited collection, historians, social scientists and philosophers reflect on definitions of antisemitism and Islamophobia in both the past and the present. Its contributors investigate the different historical contexts which have shaped definitions and examine their different political purposes and meanings, as well as addressing contemporary debates, and identifying ways for us to move beyond our current impasse. This book therefore provides a broad and new perspective from which to comprehend present day minority politics.