http://www.ilga.gov/legislation/ilcs/documents/072000050K33A-1.htm
(c) Definitions.
(1) "Armed with a dangerous weapon". A person is considered armed with a dangerous weapon for purposes of this Article, when he or she carries on or about his or her person or is otherwise armed with a Category I, Category II, or Category III weapon.
(2) A Category I weapon is a handgun, sawed-off shotgun, sawed-off rifle, any other firearm small enough to be concealed upon the person, semiautomatic firearm, or machine gun. A Category II weapon is any other rifle, shotgun, spring gun, other firearm, stun gun or taser as defined in paragraph
(a) of Section 24-1 of this Code, knife with a blade of at least 3 inches in length, dagger, dirk, switchblade knife, stiletto, axe, hatchet, or other deadly or dangerous weapon or instrument of like character. As used in this subsection (b) "semiautomatic firearm" means a repeating firearm that utilizes a portion of the energy of a firing cartridge to extract the fired cartridge case and chamber the next round and that requires a separate pull of the trigger to fire each cartridge.
(3) A Category III weapon is a bludgeon, black-jack, slungshot, sand-bag, sand-club, metal knuckles, billy, or other dangerous weapon of like character.
(Source: P.A. 91-404, eff. 1-1-00; 91-696, eff. 4-13-00.)
I believe the context is using the knife in the commission of a crime but I'm not 100% certain on that. It may only take a PO with an attitude or an aversion to knives to give you a difficult time if s/he is not completely aware of what the state or local laws are.
