Schema

1. A minimal and specialized ontology, i.e., a list of questions, answers to which describe what exists in the world. This includes only what is required for some narrow range of actions; e.g., a library card catalogue schema asks librarians only to provide enough information about the book to help library users decide if they want to browse through it, and if so, how to find it. By contrast, an ontology enables a much broader range of actions, e.g., all of those normally associated with a working trade or profession.
2. In formal logic, a rule (usually recursively definable) describing a set (usually infinite) of statements. For example, the axiom schema of replacement is a schema of axioms in axiomatic set theory.
3. A description of the structure of a database; or: a defined part of a database. See software architecture, conceptual schema, Sowa's conceptual graph, semantic network, Berners-Lee's semantic web.
4. Part of a formal specification written in the Z formal specification language.