Explanation:

Zero population growth, sometimes abbreviated ZPG (also called the replacement level of fertility), is a condition of demographic balance where the number of people in a specified population neither grows nor declines, considered as a social aim by some. According to some, zero population growth, perhaps after stabilizing at some optimum population, is the ideal towards which countries and the whole world should aspire in the interests of accomplishing long-term environmental sustainability.What it means by ‘the number of people neither grows nor declines’ is that births plus in-migrants equal deaths plus out-migrants.