Does the 'Muslim World' signify a geopolitical bloc, a civilizational unit, or a theological ideal? This Element interrogates the concept of the Muslim World as a persistent yet under-theorized category in International Relations (IR). Although widely invoked in policy discourse, academic literature, and public debate, the term often functions as a geopolitical shorthand that essentializes Muslim-majority societies and obscures their internal diversity. Rather than accepting or rejecting the term outright, this Element offers a critical reconstruction. Drawing on constructivist IR theory, postcolonial studies, and Islamic intellectual traditions, we reconceptualize the Muslim World as a transnational public sphere shaped by shared debates, symbols, institutions, and histories that generate varying degrees of referential coherence across societies. By treating the Muslim World as historically contingent, internally plural, and relational rather than fixed or monolithic, this Element advances the agenda of Global IR and the project of 'worlding beyond the West.'