Old Glory has become a common term used for the United States of America's flag. The term was first used by sea Captain William Driver and originally referred to his own flag, now an important American artifact and on display at the Smithsonian after a final stint at the Tennessee State Museum in 2006.
The original "Old Glory" was 10' by 17' and of heavy construction. It was given to Captain Driver in 1824 by his mother and a few other ladies from Salem, MA. He named the flag Old Glory in 1831 when he left on a voyage around the world on the whaling ship Charles Doggett. Old Glory was the official flag of the voyage.   Captain Driver retired to Nashville, Tennessee in 1837.
Many years later, during the Civil War, Driver still lived in Tennessee and feared that his flag would be destroyed by confederate soldiers. In order to prevent this, the flag was sewn inside a comforter and hidden within his home. When the Union forces regained Nashville in 1862, Old Glory was revealed and flown from the Capitol's flagpole. The 6th Ohio Regiment was so moved that they took "Old Glory" for their motto.  This would be the last time "Old Glory" would fly from any flagpole.  It was retired to be on display in museums thereafter.  
As the story of this victory and the symbol of one nation spread, the term Old Glory was adopted in reference to the flag of the United States.