A relational database is a database

Database

A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...

 that conforms to relational model

Relational model

The relational model for database management is a database model based on first-order predicate logic, first formulated and proposed in 1969 by Edgar F...

 theory. The software used in a relational database is called a relational database management system (RDBMS). Colloquial use of the term "relational database" may refer to the RDBMS software, or the relational database itself. A relational database is the predominant choice in storing data, over other models like the hierarchical database model or the network model

Network model

The network model is a database model conceived as a flexible way of representing objects and their relationships. Its distinguishing feature is that the schema, viewed as a graph in which object types are nodes and relationship types are arcs, is not restricted to being a hierarchy or lattice.The...

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The term relational database was originally defined by and is attributed to Edgar Codd at IBM Almaden Research Center in 1970.

Relational database theory uses a set of mathematical terms, which are roughly equivalent to SQL

SQL

SQL is a programming language designed for managing data in relational database management systems ....

 database terminology.