Most people who regularly use the Internet will be familiar with words like “misinformation,” “fake news,” “disinformation,” and maybe even “malinformation.” It can appear as though these terms are used interchangeably, and they often are. However, they don’t always refer to the same types of content, and just because a news story or social media post is false doesn’t always mean it’s problematic. To add to the confusion, not all misinformation researchers agree on the definition of the problem, or employ a unified terminology. This chapter discusses the terminology around misinformation, guided by illustrative examples of problematic news content. It also looks at what misinformation isn’t: what makes a piece of information “real” or “true”? Finally, we explore how researchers have defined misinformation and how these definitions can be categorized, before presenting the working definition that is used throughout this book.