This document defines key terms related to population: population refers to the people living in a country. The birth rate is the number of births per 1000 people in a year, while the death rate is the number of deaths per 1000 people in a year. Natural increase occurs when the birth rate is higher than the death rate, while natural decrease occurs when the death rate is higher than the birth rate. Formulas are provided to calculate birth rate, death rate, and natural increase or decrease.