A virtual population is a group of entities which share the nonphysical membership attributes of a real population and do not share the physical membership attributes of a population.
For example, citizens of the city of New York tend to have a set of psychological attributes (or, more likely, behavioral attributes) in common with one another. They perhaps tend to be fast, or have culture. This is not to say that people from Boise, Idaho do not have culture; this is merely to say that having culture is something that one would expect someone who lives in a metropolis such as New York to have. Now, let's say that we find a person living in Boise, Idaho who has culture. This person is a member of New York's virtual population by virtue of that lone attribute. If this person shares even more of the nonphysical attributes of the New Yorker population, they are said to be a member of New York's virtual population.
People who are from New York for their entire lives, but only lived there for some part of it, are members of New York's virtual population.
You can insert any city name in the examples.