Source : Free On-Line Dictionary of Computing

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)