Plagiarism is a major problem for research. There are, however, divergent views on how to define plagiarism and on what makes plagiarism reprehensible. In this paper we explicate the concept of "plagiarism" and discuss plagiarism normatively in relation to research. We suggest that plagiarism should be understood as "someone using someone else's intellectual product (such as texts, ideas, or results), thereby implying that it is their own" and argue that this is an adequate and fruitful definition. We discuss a number of circumstances that make plagiarism more or less grave and the plagiariser more or less blameworthy. As a result of our normative analysis, we suggest that what makes plagiarism reprehensible as such is that it distorts scientific credit. In addition, intentional plagiarism involves dishonesty. There are, furthermore, a number of potentially negative consequences of plagiarism.