World population
World population is the number of people alive at a reference point. It is not counted globally in real time. It is estimated from national data, censuses, registers and projections.
The number does not describe age structure, density, wealth, resource use or living conditions by itself.
Fertility and births
Fertility usually means the average number of children per woman. Births are absolute events in a period. A country can have falling fertility and still many births if many people are in parent ages.
This distinction is essential for understanding why population growth can continue after fertility starts falling.
Mortality and life expectancy
Mortality describes death patterns. Life expectancy summarizes how long a newborn would live on average if current mortality conditions applied throughout life.
It is not a promise for an individual person. It is a population-level measure.
Migration
Migration changes the population of countries and regions, but not the world total. Globally, migration redistributes people; births and deaths change the sum.
At the country level, migration can be decisive, especially where fertility is low or the population is aging.
Projection, forecast and scenario
A projection extends data under assumptions. A forecast is often read as the most likely estimate. A scenario explores what could happen under specific conditions.
Population pages should not mix these terms casually. UN variants, for example, depend on assumptions about fertility, mortality and migration.
Population momentum
Population momentum is the tendency of a population to keep growing because of its age structure even after fertility declines. Large young cohorts become future parent cohorts.
The term explains why demographic change is slow and why today’s age pyramid matters for tomorrow’s population.
Glossary status June 29, 2026: Definitions are written for education and routed to source and method pages where values need data support.
UN World Population Prospects Our World in Data population growth World population data quality
Related pages
FAQ
What is the difference between fertility and births?
Fertility is a rate or average childbearing measure. Births are absolute events in a period.
Is a projection a guaranteed forecast?
No. A projection shows what follows under specific assumptions.
Why does migration not change world population?
Because moving from one country to another changes distribution, not the global total.