Islamophobia is an emerging comparative concept in the social sciences.Yet there is no widely accepted definition of Islamophobia that permits systematic comparative and causal analysis. This article explores how the term Islamophobia has been deployed in public and scholarly debates, emphasizing that these discussions have taken place on multiple registers. It then draws on research on concept formation, prejudice, and analogous forms of status hierarchies to offer a usable social scientific definition of Islamophobia as indiscriminate negative attitudes or emotions directed at Islam or Muslims. The article discusses the types of indicators that are most appropriate for measuring Islamophobia as well as the benefits of concept development for enabling comparative and causal analysis.