As used in this act:

    (a) The terms “city” and “village” mean any city and any village in the state of Michigan existing or operating under a special or home rule charter as well as any under a general charter; and

    (b) The term “electric revenue certificates” means evidences of obligations, incurred by any city or village for or to provide funds for the extension or improvement of its electric plant or system, which are payable solely from the revenues derived from the operation of said plant or system, in whatever form.