This article combines two things: it explores how one should undertake the project of defining ‘fundamentalism’ and, based on the ensuing desiderata, it actually provides such a definition. After a few preliminary comments on ‘fundamentalism’ and the value of defining it, five goals of definitions are distinguished and elucidated: accuracy, precision, fairness, clarity, and fecundity. After that, various kinds of definitions and their interrelations are spelled out. Finally, the author provides and defends a so-called explicative definition of ‘fundamentalism’ both in terms of necessary and sufficient conditions and in terms of stereotypical properties. On the basis of empirical literature and a scoping review, it is argued that a movement is fundamentalist if and only if it is (i) reactionary towards modern developments, (ii) itself modern, and (iii) based on a grand historical narrative. More specifically, a movement is fundamentalist if it exemplifies a large number of the following properties: (i) it is reactionary in its rejection of liberal ethics, science, or technological exploitation; (ii) it is modern in seeking certainty and control, embracing literalism and infallibility about particular scriptures, actively using media and technology, or making universal claims; and (iii) it presents a grand historical narrative in terms of paradise, fall, and redemption, or cosmic dualism.