密度 (mì dù): density

This city has a very high population density, with over ten thousand people living per square kilometer.

Gold has a greater density than iron, so the same volume of gold weighs much more.

During the morning rush hour, the density of people in the subway car is so high that it's hard even to turn around.

This forest has a low tree density, so sunlight can fully reach the ground.

Scientists determine the composition of a rock by measuring its density.