A safe expression in relational calculus is one that is guaranteed to yield

Explanation

Relational calculus (both tuple and domain calculus) is expressive enough to write formulas whose result could, in principle, be an infinite set of tuples (e.g. using negation over an unbounded domain). A calculus expression is called safe precisely when it is restricted/defined so that it is guaranteed to always evaluate to a finite number of tuples, drawn only from values appearing in the database or the query itself. This restriction to safe expressions is what makes relational calculus equivalent in expressive power to relational algebra.