WordNet (r) 3.0 (2006):

half eagle
    n 1: a former gold coin in United States worth 5 dollars

Bouvier's Law Dictionary, Revised 6th Ed (1856):

HALF EAGLE, money. A gold coin of the United States, of the value of five
dollars. It weighs one hundred and twenty-nine grains. Of one thousand
parts, nine hundred are of pure gold, and one hundred of alloy. Act of
January 18, 1837, 4 Sharsw. cont. of Story's L. U. S. 2523, 4. Vide Money.