According to quindlen what notion united american culture into a single whole?

The notion that united americans' different backgrounds and cultures in a single one is patriotism. She defines it as "taking pride in this unlikely ability to throw all of us together in a country that across its length and breadth is as different as a dozen countries, and still be able to call it by one name." It's what keeps America a single country, despite the different origins and backgrounds of its inhabitants.