The Topic: Demography is "the study of statistics of births, deaths, disease, etc., as illustrating conditions of life in communities." Its subject matter is how and why the populations of particular places change over time, at least insofar as those changes can be quantified with some reasonable degree of precision. Near-synonyms: Population; Immigration; Socio-Economic Data.
The Context: New York City's population has remained largely unchanged over the past half century while the US population has doubled. Since 1970, New York City's American-born adult population has declined sharply, and is being replaced on a virtually one-for-one basis by immigrants and their children. In 1990, 28% of the city's population was foreign born. By the year 2000, the comparable figure will likely be closer to 45%.