Little attention is being paid to the implicit theories of different dimensions of religiosity that would take into account the subjective side of understanding of the phenomenon. Therefore, in the present study, lay definitions of religious fundamentalism were investigated among young adults in Slovakia. Participants were recruited by the snowball method of sampling via the Internet and they were asked to write down their own personal definitions of religious fundamentalism. Our sample consisted of 50 adults (44% females and 56% males) aged between 19 and 60 years (AM = 28.5). Regarding education, 74% were university graduates, 14% were undergraduates, and 12% had secondary education. Members of a non-specified religious group formed 62% of the sample, 26% were not members of any such group, and 12% were formal but nonpracticing members of their group. The data were analyzed by the Constant Comparison Method (Grounded Theory Methodology). The results showed the category structure of the notion which yielded formal and content categories. Within the content categories, Extremism (Extremism in belief and in behavior), Dealing with otherness (Attitudes toward otherness and Reactions to otherness), and Personality traits, states, and needs emerged. The discussion bears on a comparison between personal understanding of religious fundamentalism and previous research findings based on a review of the pertinent literature.