Density — the degree of compactness or amount per unit area/volume (e.g., population density, material density).
東京都心の人口密度は非常に高い。
The population density in central Tokyo is very high.
この金属は密度が大きいので重い。
This metal is heavy because it has a large density.
材料の密度を測定して、品質管理を行った。
We measured the material's density for quality control.