35 ILCS 625/2
As used in this Section, the following words and phrases shall have the meanings ascribed herein unless the context clearly requires otherwise:
"Department" means the Department of Revenue of the State of Illinois.
"Director" means the Director of Revenue for the Department of Revenue of the State of Illinois.
"Taxpayer" means a person engaged in the business of distributing, supplying, furnishing or selling water for use or consumption and not for resale or distributing, supplying, furnishing or selling water for use or consumption and providing sewerage disposal service.
"Person" means any natural individual, firm, trust, estate, partnership, association, joint stock company, joint adventure, corporation, limited liability company, or a receiver, trustee, conservator or other representative appointed by order of any court, or any city, town, county or other political subdivision of this State.
"Water company" means and includes every corporation, company, association, joint stock company or association, firm, partnership or individual, their lessees, trustees, or receivers appointed by any court whatsoever that is regulated by the Illinois Commerce Commission under the Public Utilities Act and that owns, controls, operates, or manages, within this State, directly or indirectly, for public use, any plant, equipment or property used or to be used for or in connection with, or owns or controls any franchise, license, permit or right to engage in:
"Water company" does not include, however, water companies, as defined in this Section, that are owned and operated by any political subdivision or municipal corporation of this State, or owned by such political subdivision or municipal corporation and operated by any of its lessees or operating agents, or which are purely mutual concerns, having no rates or charges for services, but paying the operating expenses by assessment upon the members of such a company and no other person.
"Invested capital" means that amount equal to (i) the average of the balances at the beginning and end of the taxable period of the taxpayer's total stockholder's equity and total long-term debt, less investments in and advances to all corporations, as set forth on the balance sheets included in the taxpayer's annual report to the Illinois Commerce Commission for the taxable period; (ii) multiplied by a fraction determined under Sections 301 and 304(a) of the Illinois Income Tax Act and reported on the Illinois income tax return for the taxable period ending in or with the taxable period in question. However, notwithstanding the income tax return reporting requirement stated above, beginning July 1, 1979, no taxpayer's denominators used to compute the sales, property or payroll factors under subsection (a) of Section 304 of the Illinois Income Tax Act shall include payroll, property or sales of any corporate entity other than the taxpayer for the purposes of determining an allocation for the invested capital tax. Public Act 82-1024 is not intended to and does not make any change in the meaning of any provision of this Act, it having been the intent of the General Assembly in initially enacting the definition of "invested capital" to provide for apportionment of the invested capital of each company, based solely upon the sales, property and payroll of that company.
"Taxable period" means each period which ends after August 14, 1979 and which is covered by an annual report filed by the taxpayer with the Illinois Commerce Commission.
35 ILCS 625/2