The words defined in this section shall have the meanings indicated when used in this Part:
(1) "Agreement" means any written agreement authorized by this Part.
(2) "Annexation" means any extension of a city's corporate limits as authorized by this Article, the charter of the city, or any local act applicable to the city, as such statutory authority exists now or is hereafter amended.
(3) "Participating city" means any city which is a party to an agreement.