This scoping review of conceptualizations of fundamentalism scrutinizes the concept's domain of application, defining characteristics, and liability to bias. We find fundamentalism in four domains of application: Christianity, other Abrahamic religions, non-Abrahamic religions, and non-religious phenomena. The defining characteristics which we identify are organized into five categories: belief, behavior, emotion, goal, and structure. We find that different kinds of fundamentalisms are defined by different characteristics, with violent and oppressive behaviors, and political beliefs and goals being emphasized for non-Christian fundamentalisms. Additionally, we find that the locus of fundamentalism studies is the Global North. Based on these findings, we conclude that the concept is prone to bias. When conceptualizing fundamentalism, three considerations deserve attention: the mutual dependency between the domain of application and the specification of defining characteristics; the question of usefulness of scientific concepts; and the connection between conceptual ambiguity and the risk of bias in the study of fundamentalism.