CIF dictionaries are machine-readable collections of data-name definitions and attribute sets. The core CIF dictionary includes definitions of items describing a single-crystal structure determined by diffraction methods, and the metadata needed to describe the experimental set-up, refinement strategy, and subsequent publication or database deposition details. Individual dictionaries contain definitions for other types of experiment, structure determination methodology, raw data collection and characterization, or model interpretation. Dictionaries covering most areas of chemical crystallography are maintained by COMCIFS, a committee of the International Union of Crystallography. Dictionaries specific to biological macromolecular studies are maintained by the Worldwide Protein Data Bank (wwPDB). Conventions are presented that allow individual authors or research groups to create their own data names without colliding with definitions in the COMCIFS or wwPDB dictionaries. Collectively, these dictionaries define an ontology, that is, a set of explicit formal specifications of the terms in a domain and the relations among them.