735 ILCS 5/3-101
For the purpose of this Act: "administrative agency" means a person, body of persons, group, officer, board, bureau, commission or department (other than a court or judge) of the State, or of any political subdivision of the State or municipal corporation in the State, having power under law to make administrative decisions.
"Administrative decision" or "decision" means any decision, order or determination of any administrative agency rendered in a particular case, which affects the legal rights, duties or privileges of parties and which terminates the proceedings before the administrative agency. In all cases in which a statute or a rule of the administrative agency requires or permits an application for a rehearing or other method of administrative review to be filed within a specified time (as distinguished from a statute which permits the application for rehearing or administrative review to be filed at any time before judgment by the administrative agency against the applicant or within a specified time after the entry of such judgment), and an application for such rehearing or review is made, no administrative decision of such agency shall be final as to the party applying therefor until such rehearing or review is had or denied. However, if the particular statute permits an application for rehearing or other method of administrative review to be filed with the administrative agency for an indefinite period of time after the administrative decision has been rendered (such as permitting such application to be filed at any time before judgment by the administrative agency against the applicant or within a specified time after the entry of such judgment), then the authorization for the filing of such application for rehearing or review shall not postpone the time when the administrative decision as to which such application shall be filed would otherwise become final, but the filing of the application for rehearing or review with the administrative agency in this type of case shall constitute the commencement of a new proceeding before such agency, and the decision rendered in order to dispose of such rehearing or other review proceeding shall constitute a new and independent administrative decision. If such new and independent decision consists merely of the denial of the application for rehearing or other method of administrative review, the record upon judicial review of such decision shall be limited to the application for rehearing or other review and the order or decision denying such application and shall not include the record of proceedings had before the rendering of the administrative decision as to which the application for rehearing or other administrative review shall have been filed unless the suit for judicial review is commenced within the time in which it would be authorized by this Act to have been commenced if no application for rehearing or other method of administrative review had been filed. On the other hand, if the rehearing or other administrative review is granted by the administrative agency, then the record on judicial review of the resulting administrative decision rendered pursuant to the rehearing or other administrative review may consist not only of the record of proceedings had before the administrative agency in such rehearing or other administrative review proceeding, but also of the record of proceedings had before such administrative agency prior to its rendering of the administrative decision as to which the rehearing or other administrative review shall have been granted. The term "administrative decision" or "decision" does not mean or include rules, regulations, standards, or statements of policy of general application issued by an administrative agency to implement, interpret, or make specific the legislation enforced or administered by it unless such a rule, regulation, standard or statement of policy is involved in a proceeding before the agency and its applicability or validity is in issue in such proceeding, nor does it mean or include regulations concerning the internal management of the agency not affecting private rights or interests.
735 ILCS 5/3-101