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> ## Terminology

> The following list defines some of the terms referred to in this chapter.

> - Abstract schema: The persistent schema abstraction (persistent entities, their state, and their relationships) over which queries operate. The query language translates queries over this persistent schema abstraction into queries that are executed over the database schema to which entities are mapped.
> - Abstract schema type: All expressions evaluate to a type. The abstract schema type of an entity is derived from the entity class and the metadata information provided by Java language annotations.
> - Backus-Naur Form (BNF): A notation that describes the syntax of high-level languages. The syntax diagrams in this chapter are in BNF notation.
> - Navigation: The traversal of relationships in a query language expression. The navigation operator is a period.
> - Path expression: An expression that navigates to a entity's state or relationship field.
> - State field: A persistent field of an entity.
> - Relationship field: A persistent relationship field of an entity whose type is the abstract schema type of the related entity.