Administrative law is the exercise of government authority by the executive branch and its agencies. Under the delegation doctrine, the legislative branch passes laws called "enabling acts," which give authority to agencies to enact regulations to interpret and administer statutes. These administrative regulations (often referred to as "rules") have the same force of law as statutes. 

Black's Law Dictionary defines administrative law as follows: 

The law governing the organization and operation of administrative agencies (including executive and independent agencies) and the relations of administrative agencies with the legislature, the executive, the judiciary, and the public.

Administrative law is divided into three parts: (1) the statutes endowing agencies with powers and establishing rules of substantive law relating to those powers; (2) the body of agency-made law, consisting of administrative rules, regulations, reports, or opinions containing findings of fact, and orders; and (3) the legal principles governing the acts of public agents when those acts conflict with private rights.

“Administrative law deals with the field of legal control exercised by law-administering agencies other than courts, and the field of control exercised by courts over such agencies.” Felix Frankfurter, The Task of Administrative Law, 75 U. Pa. L. Rev. 614, 615 (1927).

“[A]dministrative law is to labor law, securities regulation, and tax what civil procedure is to contracts, torts, and commercial law. Administrative law studies the way government institutions do things. It is therefore the procedural component to any practice that affects or is affected by government decision makers other than just the courts. Its study goes beyond traditional questions; it explores a variety of procedures and it develops ideas about decision-making and decisionmakers.” 1 Charles H. Koch, Administrative Law and Practice § 1.2, at 2 (2d ed. 1997).

Under the authority of the enabling act agencies enact and enforce legally binding regulations. One common means of challenging agency action is asserting that the agency exceeded the authority delegated to it in the enabling act.  

The United States and each state has an Administrative Procedure Act.

The federal Administrative Procedure Act is codified at 5 U.S.C. §§ 500 et seq., and it governs how federal administrative agencies may propose and enact administrative regulations. It also sets up a review process, allowing U.S. federal courts to directly review agency decisions.