The term THEY FILL PERIMETERS can be solved by the crossword solution AREAS. Areas are geographical regions that are defined by their boundaries or perimeters. They are typically used to describe the physical characteristics of a place, such as its climate, terrain, and vegetation. Areas can also be used to describe the population of a region, such as its size, density, and demographics. Areas are often used to divide up a larger region into smaller, more manageable parts.