The Collaborative International Dictionary of English v.0.48:

Independence Day \In`de*pend"ence Day\
   In the United States, a holiday, the 4th of July,
   commemorating the adoption of the Declaration of Independence
   on that day in 1776.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Independence Day
    n 1: a legal holiday in the United States [syn: Independence
         Day, Fourth of July, July 4]