Entities present within a set. The world population is the number of entities living on the Earth. The population of Vancouver is the number of people living in that city.

Doesn't just have to apply to people, though. Everything has a population of nodes. A beach has a population of crabs, or of grains of sand for that matter.

In statistics, a population is everything in a set. So, if you wanted to study red marbles, your population would be every red marble in the entire universe. Populations are often treated as theoretically infinite in statistics though in practice they need only be very large.

Population is a term used to describe a group of individuals of the same species that live in the same place at the same time. In terms of evolution, a population is the smallest unit that can evolve. For this reason, populations are often reffered to as the units of evolution.

An entire species does not have to change for evolution to occur. One population of that species may evolve differently than another, the study of which is called population genetics.

Pop`u*la"tion (?), n. [L. populatio: cf. F. population.]

1.

The act or process of populating; multiplication of inhabitants.

2.

The whole number of people, or inhabitants, in a country, or portion of a country; as, a population of ten millions.