510 ILCS 5/2.11a
"Enclosure" means a fence or structure of at least 6 feet in height, forming or causing an enclosure suitable to prevent the entry of young children, and suitable to confine a vicious dog in conjunction with other measures that may be taken by the owner or keeper, such as tethering of the vicious dog within the enclosure. The enclosure shall be securely enclosed and locked and designed with secure sides, top, and bottom and shall be designed to prevent the animal from escaping from the enclosure. If the enclosure is a room within a residence, it cannot have direct ingress from or egress to the outdoors unless it leads directly to an enclosed pen and the door must be locked. A vicious dog may be allowed to move about freely within the entire residence if it is muzzled at all times.
510 ILCS 5/2.11a