I love this new term I have recently run into: Peak People. It refers to a time when the world’s population reaches a maximum, after which it steadily declines due to reduced birth rates (possibly due to disease or pandemic) or global shortages of energy, food, and water. The phrase is a take on the controversial term peak oil, coined in 1956 by the geologist M. King Hubbert.
Many would correlate Peak People with Peak Oil and say that both will coincide at a time when the global rate of oil extraction reaches a maximum. The results thereafter would be both energy and population declines which would steadily lead to either a paradise of sustainability or a nightmare of barbarism (hence the controversy).