510 ILCS 5/2.05a

"Dangerous dog" means (i) any individual dog anywhere other than upon the property of the owner or custodian of the dog and unmuzzled, unleashed, or unattended by its owner or custodian that behaves in a manner that a reasonable person would believe poses a serious and unjustified imminent threat of serious physical injury or death to a person or a companion animal or (ii) a dog that, without justification, bites a person and does not cause serious physical injury.