The concept of set abstraction is introduced as a simple analogy of that of lambda abstraction in the theory of lambda calculus. The set abstraction is concerned with two extensions concerning Prolog language features: ``set expression'' and ``predicate variable''. It has been argued in the literature that the set expression extension to Prolog does really contribute to the power of the language, while the extension of predicate variables does not add anything to Prolog. Combining these two concepts of extensions to Prolog, we define ``set abstraction'' as the set expression in which predicate variables are allowed as data objects. In other words, the set abstraction gets involved in the higher order predicate logic. By showing some application examples, it is demonstrated that with the help of predicate variables set abstractions can nicely handle the issues of the second order predicate logic. Further, the implementation programs written in Prolog and Concurrent Prolog are presented.