A "prosperous country" is a place where people have enough money and resources to live well. It means the country is successful and its citizens have a good quality of life. Full definition
In prosperous countries in particular, it makes sense for central government to establish the necessary infrastructure for flood protection. (sciencedaily.com)
The science academies of the world's most prosperous countries agree, in theory. (dotearth.blogs.nytimes.com)
Especially if wages and working conditions differ in the countries included, pressure is downward in the more prosperous country. (religion-online.org)