Discontent with the term "special libraries" has pervaded the library and information field since the formation of the Special Library Association and the term's widely adopted usage in 1909. The literature related to special libraries has reflected this debate over the precise meaning and nature of special libraries for just as many years. This project considers the historical dialog regarding the varied definitions of special libraries that librarians and information professionals have been engaged in for more than a century. Using systematic review, the scholarly and professional literature is examined and analyzed to track definitional and descriptive characteristics of special libraries in the United States to identify how and why changes have occurred over time. Results reveal strong correlations between definition changes and shifting movements in broader library and United States history, especially in relation to the emergence and application of technological advances. Along with encouraging renewed discussion about the boundaries of what we in the field consider special librarianship, this study confirms that rather than permanent and rigid definitions, special libraries have always had dynamic definitions that react to the changing technologies and practices of the profession.