20 CFR 404.1242 - Back pay.
(a)Back pay defined. Back pay is pay received in one period of time which would have been paid in a prior period of time except for a wrongful or improper action taken by an employer. It includes pay made under Federal or State laws intended to create an employment relationship (including situations where there is unlawful refusal to hire) or to protect an employee's right to wages.
(b)Back pay under a statute. Back pay under a statute is a payment by an employer following an award, determination or agreement approved or sanctioned by a court or administrative agency responsible for enforcing a Federal or State statute protecting an employee's right to employment or wages. Examples of these statutes are:
(1) National Labor Relations Act or a State labor relations act;
(2) Federal or State laws providing reemployment rights to veterans;
(3) State minimum wage laws; and
(4) Civil Rights Act of 1964.
(c)Back pay not under a statute. Where the employer and the employee agree on the amount payable without any award, determination or agreement approved or sanctioned by a court or administrative agency, the payment is not made under a statute. This back pay cannot be allocated to prior periods of time but must be reported by the employer for the period in which it is paid.