What is Adjective Law?

Legal Definition
The aggregate of rules of procedure or practice. As opposed to that body of law which the courts are established to administer, (called "substantive law,") it means the rules according to which the substantive law is administered. That part of the law which provides a method for ehforcing or maintaining rights or obtaining redress for their invasion.
-- Black's Law Dictionary
Legal Definition
Law regulating procedure.
-- Ballentine's Law Dictionary