430 ILCS 155/2

As used in this Act "multiple dwelling" means any dwelling that is either rented, leased, let or hired out, to be occupied, or is occupied as the residence or home of 3 or more families living independently of each other. A "multiple dwelling" shall not be deemed to include a hospital, convent, monastery, asylum or public institution, or a fireproof building used wholly for commercial purposes except for not more than one janitor's apartment and not more than one penthouse occupied by not more than 2 families. However, residential quarters for members or personnel of any hospital staff which are not located in any building primarily used for hospital use, shall be deemed to be a "multiple dwelling".