WordNet (r) 3.0 (2006):

half dollar
    n 1: a United States coin worth half of a dollar [syn: half
         dollar, fifty-cent piece]

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

HALF DOLLAR, money. A silver coin of the United States of the value of fifty
cents. It weighs two hundred and six and one-fourth grains. Of one thousand
parts, nine hundred are of pure silver, and one hundred of alloy. Act of
January 18, 1837, S. 8 and 9, 4 Sharsw. cont. of Story's L. U. S. 2523, 4.
Vide Money.