This chapter shows why defining “fake news” is worthwhile and what a suitable definition of “fake news” might look like. It begins by introducing the authors’ definition of “fake news” (§2) and employs it to set fake news apart from related phenomena that are often conflated with it (§3). It then extracts seven potential dimensions of the concept of fake news from the literature (§4) and compares the most representative definitions that have been proposed so far along those dimensions (§5). The chapter’s primary aims are (i) to enable a systematic evaluation of prevalent definitions with respect to their extensional scope, practical utility, and conceptual transparency, (ii) to demonstrate that there is more widespread agreement than one would think at the outset, and (iii) to show (in §6) that defining “fake news” is not only far from futile, but of vital importance to confront the epistemic threats posed by fake news.