den·si·ty 
/ˈdɛnsəti/ noun
plural den·si·ties /ˈdɛnsəti/ noun 1 [noncount] : the quality or state of being dense: such as a : the quality of having parts that are close together
▪ the jungle's density
b : the quality of being difficult to see through
▪ We were surprised by the fog's density.
c : the quality of being difficult to understand
▪ the density of her writing style 2 : the amount of something in a particular space or area [count] ▪ This part of the country has a high population density. [=many people live in this part of the country] [noncount] ▪ There has been an increase in (population) density in this area. 3 technical : the amount of matter in something that is shown by the relationship between its weight and size [count] ▪ metals with different densities ▪ These instruments are used for measuring the density of the atmosphere. [noncount] ▪ older women who have lost bone density [=whose bones are less solid and heavy]
▪ the jungle's density
b : the quality of being difficult to see through
▪ We were surprised by the fog's density.
c : the quality of being difficult to understand
▪ the density of her writing style
