Population momentum is a term used to describe the tendency of a population to continue to grow despite the reduction of birth rates. It refers to the inertia of population growth that results from past increases in fertility rates. Synonyms for population momentum include demographic inertia, demographic persistence, and demographic momentum. Another synonym for this concept is the population lag effect, which recognizes the time lag between changes in fertility rates and their effects on population growth. Regardless of the term used, population momentum is an important phenomenon to understand as it can have significant implications for social, economic, and environmental issues.