Big data has been widely discussed. The diverse impacts and potential of big data have been pinpointed and empirically proven. Nevertheless, there is no consensus on the understanding of big data. Big data has been used to refer to different things and its characteristics are not universally accepted either. Therefore, this study aims to generate an overall understanding of big data. The domain of the study is limited to librarianship, because of its unique position in managing and utilising big data. Thus, the aim of this study is to understand big data in librarianship according to how it is defined in that profession. Articles containing definitions of big data were reviewed and 35 definitions were collected. Since the number of analysed definitions is small, a combined method was employed. Both a content analysis and a statistical description of these definitions were conducted. Five aspects are summarised based on the analysis of the collected definitions. These aspects help explicate libraries’ current understanding of big data and librarians’ big data skills.