WordNet (r) 3.0 (2006):

relational database
    n 1: a database in which relations between information items are
         explicitly specified as accessible attributes; "in a
         relational database the data are organized as a number of
         differently sized tables"

The Free On-line Dictionary of Computing (30 December 2018):

relational database
RDBMS
relational database management system
relational DBMS

    (RDBMS - relational database management system) A
   database based on the relational model developed by
   E.F. Codd.  A relational database allows the definition of
   data structures, storage and retrieval operations and
   integrity constraints.  In such a database the data and
   relations between them are organised in tables.  A table is
   a collection of rows or records and each row in a table
   contains the same fields.  Certain fields may be designated
   as keys, which means that searches for specific values of
   that field will use indexing to speed them up.

   Where fields in two different tables take values from the same
   set, a join operation can be performed to select related
   records in the two tables by matching values in those fields.
   Often, but not always, the fields will have the same name in
   both tables.  For example, an "orders" table might contain
   (customer_id, product_code) pairs and a "products" table might
   contain (product_code, price) pairs so to calculate a given
   customer's bill you would sum the prices of all products
   ordered by that customer by joining on the product-code fields
   of the two tables.  This can be extended to joining multiple
   tables on multiple fields.  Because these relationships are
   only specified at retreival time, relational databases are
   classed as dynamic database management system.

   The first commercial RDBMS was the Multics Relational Data
   Store, first sold in 1978.

   INGRES, Oracle, Sybase, Inc., Microsoft Access, and
   Microsoft SQL Server are well-known database products and
   companies.  Others include PostgreSQL, SQL/DS, and RDB.

   ["Managing Data Bases, Four Critical Factors" Michael
   M. Gorman, QED Information Sciences, Inc.].

   ["An Introduction To Database Systems" (6th ed) C. J. Date,
   Addison Wesley (an excellent source of detailed info)].

   ["An End-User's Guide to Data Base" James Martin, Prentice
   Hall (excellent place to begin learning about DBMS)].

   (2002-06-10)