The total fertility rate (TFR, sometimes also called the fertility rate, period total fertility rate (PTFR) or total period fertility rate (TPFR)) of a population is the average number of children that would be born to a woman over her lifetime if she were to experience the exact current age-specific fertility rates (ASFRs) through her lifetime. It is obtained by summing the age-specific rates for a given time-point.
Graph of Total Fertility Rates vs. GDP per capita of the corresponding country, 2004. Only countries with over 5 Million population were plotted to reduce outlyers. Sources: CIA World Fact Book
Graph of Total Fertility Rates vs. GDP per capita of the corresponding country, 2004. Only countries with over 5 Million population were plotted to reduce outlyers. Sources: CIA World Fact Book

The TFR is a synthetic rate, not something that is actually counted. It is not based on the fertility of any real group of women, since this would involve waiting until they had completed childbearing. Nor is it based on counting up the total number of children actually born over their lifetime, but instead is based on the age-specific fertility rates of women in their "child-bearing years," which in conventional international statistical usage is ages 15-44 or 15-49.

The TFR is therefore a measure of the fertility of an imaginary woman who passes through her reproductive life subject to all the age-specific fertility rates for ages 15-49 that were recorded for a given population in a given year. The TFR represents the average number of children a woman would have were she to fast-forward through all her childbearing years in a single year, under all the age-specific fertility rates for that year. In other words, this rate is the number of children a woman would have if she was subject to prevailing fertility rates at all ages from a single given year, and survives throughout all her childbearing years.