so-called slot machines

[W]e interpret this history, however, as demonstrating a congressional purpose to place a heavy tax on all 'slot-machine' gambling devices, regardless of their particular structure, and a substantially smaller tax on machines played purely for amusement which offered the player no expectation of receiving 'cash, premiums, merchandise, or tokens.' ... Congress ... meant only to distinguish between 'slot-machines' operated as gambling devices and 'slot-machines' which were used exclusively for amusement.

Defined by the Supreme Court in - United States v. Korpan, 354 U.S. 271 (1957).