The classic definition is that an antiquarian book is simply a book that was printed before 1851. But today the term refers to a larger reality: it denotes any rare book, more or less valuable, not to be found in a traditional library but offered "antiquarian bookseller."

Determining the value of an antiquarian book is based on several criteria: the author and the title of course, scarcity of publishing,  quality of the book, condition of the binding, or unique Characteristics such as an inscription or a prestigious provenance, etc.