In Guns, Germs, and Steel how does Diamond define the east-west axis and the north-south axis?
The answer to this can be found on the first page of Chapter 10 and the idea is also illustrated in Figure 10.1 on the next page. Basically, a north to south axis is the distance from the farthest northern point on a given land mass to that land mass's southernmost point. The east-west axis is the distance between the point that is farthest east and the one that is farthest west.
As you can see in Figure 10.1, the Americas have a long north-south axis since it is a very long way from the northern part of Canada to the southern tip of South America. By contrast, the Americas have a relatively short east-west axis since it is not all that far from the West Coast of the US to its East Coast.