26 CFR § 1.451-2 - Constructive receipt of income. (a) General rule. Income although not actually reduced to a taxpayer's possession is constructively received by him in the taxable year during which it is credited to his account, set apart for him, or otherwise made available so that he may draw upon it at any time, or so that he could have drawn upon it during the taxable year if notice of intention to withdraw had been given.
However, income is not constructively received if the taxpayer's control of its receipt is subject to substantial limitations or restrictions. Thus, if a corporation credits its employees with bonus stock, but the stock is not available to such employees until some future date, the mere crediting on the books of the corporation does not constitute receipt.
The IRS in Publication 538 describes constructive receipt as: “an amount [that] is credited to your account or made available to you without restriction.”