1The number of people living in a particular area or country; population (including aspects like growth or density).
都市部の人口は年々増加している。
The population of urban areas is increasing year by year.
この国の総人口は約一億人だ。
The total population of this country is about 100 million.
人口密度が高い地域では住宅問題が深刻だ。
In areas with high population density, housing problems are serious.
2Being talked about widely by the public; becoming common knowledge. In modern Japanese this sense survives almost exclusively within the fixed idiom 人口に膾炙する ("to be on everyone's lips / widely known"), and is rarely used on its own.
彼の名前は今や人口に膾炙している。
His name is now widely known to everyone.
その俳句は人口に膾炙する名作として知られている。
That haiku is known as a celebrated masterpiece that's on everyone's lips.