зичии аҳолӣ: population density
In Asia the average population density is 95 people in one square kilometre.
Зичии аҳолии Чин нисбат ба зичии аҳолии ҷаҳон якчанд маротиба зиёд аст.
The population density of China compared to the population density of the world is several times greater.
зич: (adj) dense, tightly packed, dense
зичӣ: (noun) density, compactness
зичии аҳолӣ: the density of the population
Notice that the ӣ is changed to и when the additional и is added; you can’t have the letter ӣ in the middle of a word.