The term natural change (natural increase or natural decrease) in a population is the difference between the number of births and number of deaths in the population. If the number of births is greater than the number of deaths, then we have:
natural increase = (number of births) - (number of deaths)
If the number of deaths is greater than the number of births, then we have:
natural decrease = (number of deaths) - (number of births)
The term natural in conjunction with the use of the terms change, increase, or decrease, is used to indicate that we are not considering the effects of migration on the population, at least in the short term (in the long term, since the migrants will themselves give birth to children, there will be an effect).