Source : Free On-Line Dictionary of Computing

relational calculus
     
         An operational methodolgy, founded on {predicate
        calculus}, dealing with descripitive expressions that are
        equivalent to the operations of {relational algebra}.  {Codd's
        reduction algorithm} can convert from {relational calculus} to
        {relational algebra}.
     
        Two forms of the relational calculus exist: the {tuple
        calculus} and the {domain calculus}.
     
        ["An Introduction To Database Systems" (6th ed), C. J. Date,
        Addison Wesley].
     
        (1998-10-05)