The core-periphery hierarchy in the modern world-system is a system of stratification in which socially structured inequalities are reproduced by the institutional features of the system. The periphery is not "catching up" with the core. Rather, both core and peripheral regions are developing, but most core states are staying well ahead of most peripheral states. There is also a stratum of countries that we call the semiperiphery: countries that are in between the core and the periphery.
— Global Social Change: Historical and Comparative Perspectives
by Christopher Chase-Dunn, Salvatore J. Babones