An ingame concept devised by DionysianTwilight, it classes the largest nations in the game into three rough brackets based on a combination of size and influence. The system has had to shift as the game grew, but has remained a useful way for some older nations to track their status and power.
Elder Powers are ranked primiarly not by sheer size but age-- as some of the oldest continuous governments in the game, they are assumed to have a certain degree of "weight" in technology and military prowress. This cannot trump a complete drop in one's stats, however or massive decline of a nation overall. There is some debate over whether Elder Powers that dramatically decline could lose this status, such as the case of ACCR and Perú in recent years. Nevertheless, barring a complete societal collapse, Elder Powers are still widely regarded as formidable, if not unassailable nations.