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

relational algebra

    A family of algebra with a well-founded
   semantics used for modelling the data stored in relational
   databases, and defining queries on it.  The main operations
   of the relational algebra are the set operations (such as
   union, intersection, and cartesian product), selection
   (keeping only some lines of a table) and the projection
   (keeping only some columns).

   The relational data model describes how the data is
   structured.

   Codd's reduction algorithm can convert from relational
   calculus to relational algebra.

   (1997-02-17)