I guess I always thought they just called that “Predestination.” Why the ‘Double’?
It can be called Predestination, or even Determinism used non-theologically, but to clarify theologically, Calvin’s view of predestination is called “double” because both saved and damned are predestined.
That God’s grace is withheld from some, that Christ did not die for their sins, their is no free will concerning salvation (either for saved or damned), etc. flow from or are tied up in this foundational belief.