Population density can be defined as the number of people in relation to the amount of space that surrounds them. For example, the number of people in a city block, city district , square mile, metropolis, megalopolis, region, state, country, or the entire planet, are the most common denominators of population density.
Too much population density is associated with social pathology, contributes to the overwhelming of the environment and infrastructure, and creates issues with the growth in supporting social structures to maintain order, communicable disease control, and sanitation.
Simply dividing the size of a land mass, say the State of California into the population of California which has about 37 million people in 404 thousand square miles of space, gives a population density for the state of about 236 people per square mile. But this does not mean that each and every square mile of California has exactly 236 people standing around.
America has about 76 people per square mile in population density. But not each and every square mile of America has 76 people standing around. As a result, definitions of population density expand to include other factors such as the locations within a state that have the greatest population density per square mile. These can be called "population centers".
Population centers for the United states then can be categorized as "the top ten population centers", or the "densest cities". That way, the population centers for California can be defined as the Greater Los Angeles Area, the Bay Area, and so on. Within the City of Los Angeles, a particular neighborhood may contain the greatest density.
This still means that one person will be standing in each and every square foot of space within a defined area. As a result, while population density is easy to calculate, once the space is defined, the number of people within that space are accurately counted, and one number is divided into another, it does not mean that the data is meaningful.
Comparing the population density of one state with another is more meaningful. Finding the locations in the world that have the highest or lowest population densities is more meaningful. It is important to know that most of the worlds population is concentrated in a very small proportion of the entire world's land mass.
Finally, quality and timeliness of data is critical. If the information is from the 2000 US Census, it will be far less timely than the latest Census Bureau projections from 2008. But projections are not actual facts.
It is not of the best quality when the information only states that Monaco has a population of about 35 thousand. Monaco is the most densely populated country in Europe and possibly the world, because it is one of the tiniest countries in the world! So any meaningful definition of population density should include how the density figure is explained and constructed.