What is population momentum?
Population momentum is how the population grows when reproduction is reduced to replacement-level fertility.
Sexual reproduction takes two organisms. When two organisms have two offspring, this is known as replacement-level fertility, because they are exactly replacing themselves in numbers.
Population momentum is the growth of a population if reproduction were immediately reduced to replacement-level fertility. This population growth is due to the current number of organisms in the childbearing age-range. In other words, the growth of the population based on how big it already is, rather than because of how it increases.