26 C.F.R. § 1.404(b)-1
Section 404(a) is not confined to formal stock bonus, pension, profit- sharing, and annuity plans, or deferred compensation plans, but it includes any method of contributions or compensation having the effect of a stock bonus, pension, profit-sharing, or annuity plan, or similar plan deferring the receipt of compensation. Thus, where a corporation pays pensions to a retired employee or employees or to their beneficiaries in such amounts as may be determined from time to time by the board of directors or responsible officers of the company, or where a corporation is under an obligation, whether funded or unfunded, to pay a pension or other deferred compensation to an employee or his beneficiaries, there is a method having the effect of a plan deferring the receipt of compensation for which deductions are governed by section 404(a). If an employer on the accrual basis defers paying any compensation to an employee until a later year or years under an arrangement having the effect of a stock bonus, pension, profit-sharing, or annuity plan, or similar plan deferring the receipt of compensation, he shall not be allowed a deduction until the year in which the compensation is paid. This provision is not intended to cover the case where an employer on the accrual basis defers payment of compensation after the year of accrual merely because of inability to pay such compensation in the year of accrual, as, for example, where the funds of the company are not sufficient to enable payment of the compensation without jeopardizing the solvency of the company, or where the liability accrues in the earlier year, but the amount payable cannot be exactly determined until the later year.
26 C.F.R. §1.404(b)-1