Among the conceptual foundations on which scholars of modern Islam have built their narratives for decades, ideas such as “reform” and “reformism” have been singled out for charges of Eurocentricism and Orientalism. At the same time, research on early modern Islam leads us to question the specificities of these nineteenth and twentieth-century concepts. Building on this scholarship, this article examines the case of Algerian Ibadi reform ( iṣlāḥ ) in order to reassert the specificity of the early twentieth century as a moment when Islamic concepts acquired new meanings, but also as a moment of deep entanglements between Islamic and colonial knowledge production. It shows that a systematic understanding of iṣlāḥ as social and religious reform linked to the idea of progress developed only during the interwar period. It also demonstrates that the emic and etic uses of iṣlāḥ and “reform” developed together, a result of the confluence between modern Islamic scholarship and scholarship about Islam in the early twentieth-century Algerian colonial public sphere. Thus, the conceptual history of iṣlāḥ warns us against approaches that consider emic and etic categories bounded entities and invites us instead to unravel their complexities.