Mid-20th-century baby boom in the context of "Baby Boomer"

Population decline, also known as depopulation, is a reduction in a human population size. Throughout history, Earth's total human population has continued to grow, but projections suggest this long-term trend may be coming to an end. From antiquity (10th century BCE–500 CE) until the beginning of the Industrial Revolution in early modern Europe (late 18th–early 19th centuries), the global population grew very slowly, at about 0.04% per year. After about 1800 the growth rate accelerated to a peak of 2.1% annually during the mid-20th-century baby boom (1945–1968 period), but since then, due to the worldwide collapse of the total fertility rate, it has slowed to 0.9% as of 2023. The global growth rate in absolute numbers accelerated to a peak of 92.8 million in 1990, but has since slowed to 70.4 million in 2023.

Long-term projections indicate that the growth rate of the human population on the planet will continue to slow down, and that before the end of the 21st century it will reach growth zero. Examples of this emerging trend are Japan, whose population is currently (2023) declining at the rate of 0.5% per year, and China, whose population has peaked and is currently (2023) declining at the rate of about 0.2% per year. By 2050, Europe's population is projected to be declining at the rate of 0.3% per year. Population growth has declined mainly due to the abrupt decline in the global total fertility rate, from 5.3 in 1963 to 2.2 in 2023. The decline in the total fertility rate has occurred in every region of the world and is a result of a process known as demographic transition. To maintain its population, ignoring migration, a country on average requires a minimum fertility rate of 2.2 children per woman of childbearing age (the number is slightly greater than two because not all children live to adulthood). However, most societies experience a drop in fertility to well below two as they grow wealthier.

Baby boomers, often shortened to boomers, are the demographic cohort preceded by the Silent Generation and followed by Generation X. The generation is often defined as people born from 1946 to 1964 during the mid-20th-century baby boom that followed the end of World War II. The dates, the demographic context, and the cultural identifiers may vary by country. Most boomers are the parents of Generation X and Millennials.

In the West, boomers' childhoods in the 1950s and 1960s had significant reforms in education, both as part of the ideological confrontation that was the Cold War, and as a continuation of the interwar period. Theirs was a time of economic prosperity and rapid technological progress, and many grew up expecting the world to improve with time.