From Warhammer 40k - Lexicanum
Population is a measure of the total population of a certain geographical area. Most often in Warhammer 40,000, population is a measure of an entire planet but can also be a of a hive or small community.
Certain world classes have a tendency towards larger or smaller populations, for example agri worlds tend to have lower populations as they don't have room for vast industrial hives, whereas forge worlds have a much greater population compressed into a smaller space to operate the machinery required to produce the vast amounts of materials and weapons the Imperium needs to survive.
- 1:Warhammer 40,000 3rd Edition Rulebook, pg. 114