Population density is the number of people living in a specific area. It is usually measured as people per square kilometer or square mile. This helps compare how crowded different places are.

Cities such as Tokyo have very high population density, with many people living in a small area. In contrast, deserts, mountains, and polar regions often have very low population density. Maps are often used to show these differences.

Population density is used in planning cities, services, and transportation. It is studied in geography to understand how people are distributed across the Earth.