A "rule" is defined broadly in the Administrative Procedure Act to include any agency statement of future effect designed to: (1) implement, interpret or prescribe law or policy; or (2) describe the agency's organization, procedure or practice requirements. As such, it includes nearly every type of agency pronouncement other than an order made in an adjudication-from the broadest substantive regulatory standard to the most trivial sort of procedural memorandum or guidance document. However, although the definition is quite expansive, the rulemaking process for rules differs depending on the nature of the rule being issued by the agency. To have the force of law the rule must be issued under a delegation of authority from Congress and according to appropriate rulemaking requirements. Violation of a validly adopted rule can result in a sanction just as severe as one received for violation of a statute passed by Congress.

Agencies' organizational descriptions and rules of general applicability adopted by agencies must be published in the Federal Register. Many of these rules must also be published first in proposed form, unless they fall within one of the exceptions from notice and comment. (See Rulemaking Exceptions.)

Finally, some statutes and Executive Orders require additional procedures and impact statements for certain categories of rules. (See Regulatory Flexibility Act, Unfunded Mandates Reform Act, Impact Statements, Executive Order.) The terms "regulation" and "rule" are typically used interchangeably. (See Regulation.)