What defines a schema in a database?

A schema in a database is fundamentally a blueprint that defines how data is organized and structured within the database. It outlines the tables, fields, data types, relationships, and constraints of the database objects. In essence, it serves as a framework for the structure of the records in terms of how they are formatted and how they correspond to one another.

While the other options present concepts that are related to databases, they do not define what a schema is. A collection of tables might describe part of a schema, but it does not encompass the entire structural definition included therein. An index of database queries refers to a mechanism meant to improve the speed of data retrieval and does not relate to the structural definition of data. A tool for data visualization pertains to software or methodologies used to present data graphically, rather than defining the schema or structure within the database. Thus, the correct choice highlights the specification of records within the database, which is a direct reflection of the schema's purpose.