"Population doublings" refers to the exponential growth of a population, where the number of individuals doubles repeatedly over a certain period of time. It indicates how quickly a population can increase in size. Full definition
The world's population doubled twice in the 20th century, hitting 6 billion in 1999. (sciencemag.org)
Our recent achievements are impressive - while global population doubled to 6 billion people in the 40 years from 1960, global food production more than kept up. (news.bbc.co.uk)
He said the world population doubled from 1960 to 2000, and the pace of food production more than doubled in the same period, so he expects Earth to adjust unless the planet hits either extreme. (scientificamerican.com)