Replacement Level

The level of fertility at which the existing generation produces just enough children to replace itself , so that the next generation is of the same size (total population) as the current one.
This translates to the rule of thumb that a woman needs to have approximately 2.1 children to ensure that she and her spouse are ‘replaced’ (the extra 0.1 is required to compensate for the risk of unforeseen or accidental deaths).
In other words, the replacement level of the total fertility rate is usually said to be 2.1.