Not a separate category of money under current federal law; the statutory term is “legal tender,” which includes U.S. coins and currency, including Federal Reserve notes (31 U.S.C. § 5103). A related phrase, “lawful money,” still appears in some statutes, such as the provision on redeeming Federal Reserve notes (12 U.S.C. § 411), but we have not found a court decision holding that it means only gold or silver, or that demanding “lawful money” changes a person’s tax or debt obligations; courts that have addressed these arguments, such as Milam v. United States, 524 F.2d 629 (9th Cir. 1974), ruled the other way.