A Population

1. The people living within a political or geographical boundary.
2. (by extension) The people with a given characteristic.
3. A count of the number of residents within a political or geographical boundary such as a town, a nation or the world.
4. (biology) A collection of organisms of a particular species, sharing a particular characteristic of interest, most often that of living in a given area.
5. (statistics) A group of units (persons, objects, or other items) enumerated in a census or from which a sample is drawn.
6. (computing) The act of filling initially empty items in a collection.

Physics:

State Population:

The number of atoms per unit volume in a given state.

Populations can be changed by:

• Transitional Excitation: Transition Processes
• Collisional Excitation.