Lawful money per US Code

[allodial]

Etymologically speaking:

yield (n.)
O.E. gield "payment, sum of money" (see yield (v.)); extended sense of "production" (as of crops) is first attested mid-15c. Earliest English sense survives in financial "yield from investments."

yield (v.)
O.E. geldan (Anglian), gieldan (W.Saxon) "to pay" (class III strong verb; past tense geald, p.p. golden), from P.Gmc. *geldanan "pay" (cf. O.S. geldan "to be worth," O.N. gjaldo "to repay, return," M.Du. ghelden, Du. gelden "to cost, be worth, concern," O.H.G. geltan, Ger. gelten "to be worth," Goth. fra-gildan "to repay, requite"), perhaps from PIE *ghel-to- "I pay," found only in Balto-Slavic and Germanic, unless O.C.S. zledo, Lith. geliuoti are Germanic loan-words. Sense developed in English via use to translate L. reddere, Fr. rendre, and had expanded by c.1300 to "repay, return, render (service), produce, surrender." Related to M.L.G. and M.Du. gelt, Du. geld, Ger. Geld "money." Yielding in sense of "giving way to physical force" is recorded from 1660s.