enjoyment (§ 811(d)(2))

[Section] 811(d)(2) was more concerned with 'present economic benefit' than with 'technical vesting of title or estates.' * * * 'A donor who keeps so strong a hold over the actual and immediate enjoyment of what he puts beyond his own power to retake has not divested himself of that degree of control which § 811(d)(2) requires in order to avoid the tax.

Defined by the Supreme Court in - Lober v. United States, 346 U.S. 335 (1953).