the number of people or things in a place when compared with the size of the place

The area has a high/low population density. 该地区人口密度大／小。

We were unable to move because of the density of the crowd. 人太多，我们根本就动不了。

the relationship between the mass of a substance and its size

Lead has a high density. 铅的密度大。

Aluminium is low in density. 铝的密度小。