General definition
(b) General definition. In general, the term income in respect of a decedent refers to those amounts to which a decedent was entitled as gross income but which were not properly includible in computing his taxable income for the taxable year ending with the date of his death or for a previous taxable year under the method of accounting employed by the decedent. See the regulations under section 451. Thus, the term includes: (1) All accrued income of a decedent who reported his income by use of the cash receipts and disbursements method; (2) Income accrued solely by reason of the decedent's death in case of a decedent who reports his income by use of an accrual method of accounting; and (3) Income to which the decedent had a contingent claim at the time of his death.