As used in this chapter, the term:

(1) "Corporate authorities" means the proper officials within any municipality whose duty it is to establish the wages, salaries, rates of pay, hours, working conditions, and other terms and conditions of employment of firefighters, as defined in paragraph (2) of this Code section, whether they are the mayor, city manager, city administrator, city council, board of aldermen, director of personnel, personnel board, or any combination thereof, or by whatever other name the same may be designated. The term shall also mean the employing authority for firefighters at Central State Hospital.

(2) "Firefighter" means the permanent members of any paid fire department of any municipality of this state having a population of 20,000 or more according to the United States decennial census of 1980 or any future such census who are employed for and subject to fire-fighting duties.