A data dictionary is a set of Metadata (computing) that contains definitions and representations of data elements.

Within the context of a DBMS, a data dictionary is a read-only set of tables and views. The data dictionary may be considered a Database in its own right.

Amongst other things, a data dictionary holds the following information:

One benefit of a well prepared data dictionary is a consistency between data items across different tables. For example, several tables may hold telephone numbers, using a data dictionary the format of this telephone number field will be consistent.

When an organization builds an enterprise-wide data dictionary it may include both semantics and representational definitions for data elements. The semantic components focus on creating precise meaning of data elements. Representation definitions include how data elements are stored in a computer structure such as an integer, string or date format.

Data dictionaries are one step along a pathway of creating precise semantic definitions for an organization.

Initially, data dictionaries are sometimes simply a collection of Database columns and the definitions of what the meaning and types the columns contain.

Data dictionaries are more precise than glossaries (terms and definitions) because they frequently have one or more representations of how data is structured. Data dictionaries are usually separate from data models since data models usually include complex relationships between data elements.

Data dictionaries can evolve into full ontology (computer science) when discrete logic is added to data element definitions.

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