This volume in the International Tables series, Definition and exchange of crystallographic data, deals with the precise definition of data items used in crystallography. It focuses on a particular data representation, the Crystallographic Information File (CIF), developed over the last 15 years for the computerized interchange of data important to structural crystallography, and for submissions to crystallographic publications and databases. The need for the volume stems from the IUCr's adoption in 1990of CIF as the standard for exchanging crystallographic data across the entire community, and for the submission of research articles to Acta Crystallographica and other journals. The data items described in this volume appear as standard tags in CIF. Each data item has an associated definition, with attributes that characterize its allowed values and relationships with other defined items. The definitions provide a comprehensive machinereadable description of the information associated with a structural study, from area-detector counts through to the derived molecular geometry, and allow a full annotation suitable for publication as a primary research article.