"Population momentum" is the term describing a country or other demographic unit that is continuing to gain population despite the fact that its total fertility rate is at, or below, replacement levels.
The total fertility rate is the amount of children that each woman is expected to produce in her lifetime, and a number of between 2 and 2.1 is considered to be the replacement rate. However, even when an average woman has less than two children, the population can continue to grow. This is because new children are being born while elderly citizens, who are no longer of childbearing age, are still alive. This is both marked in countries that had a bump in their population pyramid, as the United States did after World War II. Population momentum, along with immigration, is why a country with a total fertility rate less than replacement, can continue to grow in population. And for the world as a whole, population momentum is the only reason the population will continue to grow once the fertility rate drops below 2.1
Many of the world's countries are now losing population, or are only maintaining and growing population through immigration. For example, most of Western and Eastern Europe are in this condition. Of the countries that are gaining population, many of them are doing so through population momentum. Estimates on total fertility rate are not precise, and estimates differ, but some countries that are currently in a state of population momentum are China, Vietnam, Thailand, Brazil and Chile. Some countries that are nearing a state of population momentum are Mexico, South Africa, Indonesia and Argentina. In general, most of East and Southeast Asia, as well as South and Central America, which were once rapidly growing with high fertility rates, are now only growing through population momentum.
Currently, the world still has a total fertility rate over replacement levels. Demographic trends seem to suggest that within a generation, this could change. However, because of population momentum, the world will continue to gain population throughout most of this century.
Sources:
http://www.uwmc.uwc.edu/geography/demotrans/demodef.htm -- A definition of Population momentum
https://www.cia.gov/library/publications/the-world-factbook/rankorder/2127rank.html
http://www.nationmaster.com/graph/peo_tot_fer_rat-people-total-fertility-rate
http://data.worldbank.org/indicator/SP.DYN.TFRT.IN
--Three different sources on estimates of total fertility rate, by country.