I'll just read you the passage from Stossenger directly:

Surely one of the most perplexing concepts is that of "national character." Few social scientists would deny that certain cultural patters occur more frequently and are more highly valued in one nation than in another. But it is almost impossible to find agreement among scholars on precisely what these common patterns are. In other words, we are faced with the paradox that "national character" seems to be an indisputable factor but that no one knows exactly what it is. This confusion probably stems from the fact that cultural patterns continue to live as stereotypes. For example, the stereotypes of the "volatile Frenchman" and the "materialistic American" are strictly time-bound. Only a century ago almost opposite images were current. Moreover, patterns may differ from region to region in the same country. And it is never difficult to find exceptions to the prevailing images. On the whole, it would therefore appear that though national character patters are a fact, their uniqueness and their significance in supporting national unity vary from nation to nation.

Basically. "National Character" is that unique set of cultural behaviors that are time-bound, and are commonly understood by both the people within the nation about themselves, and about the people in the nation from the view of other peoples. Typically spoken as stereotypes.