It means one that does not grow or shrink, that is remains constant.
It possibly means one that does not increase, but I would say that a population that shrinks has negative growth, not zero.
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This is an idea created by Paul Ehrlich saying we are overpopulating the world and our resources will be diminished in the near future so he and his followers suggested bring the fertility rate to 2.11 this will even with the mortality rate and will eventually bring the overall population down