(a) Metropolitan commuter transportation district. The metropolitan commuter transportation district ("MCTD") means the area of the state included in the district created and governed by section twelve hundred sixty-two of the public authorities law.

(b) Employer. Employer means an employer required by section six hundred seventy-one of this chapter to deduct and withhold tax from wages, that has a payroll expense in excess of three hundred twelve thousand five hundred dollars in any calendar quarter; other than

(1) any agency or instrumentality of the United States;

(2) the United Nations;

(3) an interstate agency or public corporation created pursuant to an agreement or compact with another state or the Dominion of Canada; or

(4) Any eligible educational institution. An "eligible educational institution" shall mean any public school district, a board of cooperative educational services, a public elementary or secondary school, a school approved pursuant to article eighty-five or eighty-nine of the education law to serve students with disabilities of school age, or a nonpublic elementary or secondary school that provides instruction in grade one or above, all public library systems as defined in subdivision one of section two hundred seventy-two of the education law, and all public and free association libraries as such terms are defined in subdivision two of section two hundred fifty-three of the education law.

(c) Payroll expense. Payroll expense means wages and compensation as defined in sections 3121 and 3231 of the internal revenue code (without regard to section 3121(a)(1) and section 3231(e)(2)(A)(i)), paid to all covered employees.

(d) Covered employee. Covered employee means an employee who is employed within the MCTD.

(e) Net earnings from self-employment. Net earnings from self-employment has the same meaning as in section 1402 of the internal revenue code.