As used in this Act:
"Department" means the Illinois Department of Revenue.
"Rules" means any rules adopted or forms prescribed by the Department.
"Taxable period" means any period of time for which any tax is imposed by and owed to the State of Illinois.
"Taxpayer" means any person, corporation, or other entity subject to any tax, except for the motor fuel use tax, imposed by any law of the State of Illinois and payable to the State of Illinois.
35 ILCS 745/5