The article studies definitions of genre that exist in mass readers’ minds and appear in their book reviews. While readers’ literary criticism and other forms of online-communication per se are now beginning to draw scientific interest, the problem of genre in fiction book reviews written by readers has been underinvestigated. Non-professional literary critics have a rather vague idea of literary genres that often lacks logic and does not comply with traditional hierarchy of genres. Readers try to fit the text they have read into a genre system they understand, but if it does not fit they create or coin a new genre definition that reflects their own understanding of the text, how they define its subject matter and what the text made them think about. In the minds of novice readers genre definition is closely related to the plot and their personal opinion of the book. Genre criteria analysis is often overlooked or seems arbitrary, oftentimes readers use the first genre definition that comes to their minds, i.e. a socially-agreed-upon definition, a convenient definition, a general definition, or a definition that has been widely used lately (for example, in the movie or book advertisements). Using their own reading experience, reviewers create their own reader’s glossary, in which literary movement is intertwined with their own estimation of the book quality, stylistic devices are mixed up with the author’s individual writing style, and reader’s emotional response becomes a genre criterion.