Overpopulation means that the population of a place is too high. Specifically, there are too many organisms of a certain species in a habitat, so the number of organisms living there is larger than the carrying capacity of the habitat. The habitat cannot support these numbers over time without hurting itself.

The term "overpopulation" is most often used to refer to the number of too many humans living on Earth.

Human overpopulation
The world's population has greatly increased in the last 50 years. The main reason is the reduction in death rate, especially for infants and children. The result is that many more people survive to the age when reproduction is possible.
Reduction of death rate
By reducing the effect of infectious diseases 
widespread use of antibiotics against bacterial diseases
increased use of vaccination against some viral diseases
wider provision of clean water (modern sewage systems etc.), which reduces parasitic diseases.
By increasing food production
the world-wide use of DDT, and later anti-pest treatments.
the invention of high-yielding varieties of crops and later, genetically-engineered crops. 
Steve Jones, head of the biology department at University College London, has said, 
"Humans are 10,000 times more common than we should be, according to the rules of the animal kingdom, and we have agriculture to thank for that. Without farming, the world population would probably have reached half a million by now".

Against this background, the reduction in fertility has had little effect, except perhaps in China. The use of the contraceptive pill has transformed the lives of women in rich countries, but has made little impact in poor ones. 

The recent rapid increase in human population over the past two centuries has raised concerns that humans are beginning to overpopulate the Earth. The planet may not be able to sustain larger numbers of people. The population has been growing since the end of the Black Death, around the year 1400. At the beginning of the 19th century, it had reached roughly 1,000,000,000 (one billion). Rapid population growth occurred all over the world, especially after World War II. By 1960, the world population had reached 3 billion, and it doubled to 6 billion over the next four decades. As of 2011, the estimated annual growth rate was 1.10%, down from a peak of 2.2% in 1963, and the world population stood at roughly 6.9 billion. In 2014, it is over seven billion. 

Current projections show a steady decline in the population growth rate, with the population expected to reach between 8 and 10.5 billion between the year 2040 and 2050.

The scientific consensus is that the present population growth and increase in use of resources is a threat to the ecosystem. The InterAcademy Panel Statement on Population Growth called the growth in human numbers "unprecedented", and stated that many environmental problems, such as rising levels of atmospheric carbon dioxide, global warming, and pollution, were made worse by the population expansion. At the time, the world population stood at 5.5 billion, and optimistic scenarios predicted a peak of 7.8 billion by 2050, a number that current estimates show will be reached around 2030.

Loss of forests 
What were forests have often been turned into farmland. In so doing many species of animals and plants are wiped out, and the CO2 levels in the atmosphere goes up. This is a direct consequence of human population increase.

Potential solutions 
The solutions usually suggested are better education and widespread free contraception (birth control). Many pregnancies are unplanned (40%) or unwanted. 

There are powerful forces working against birth control. Religious and traditional beliefs often favor large families. Few governments have tackled the problem seriously.

Others have proposed that since human population size and carrying capacity depends on the amount of food available, like other species, a good method would be to control food production and produce food only for how much is needed rather than more than needed in order to stop birthrates. 

For example, Peter Farb, an anthropologist, ecologist and naturalist stated "Intensification of production to feed an increased population leads to a still greater increase in population."

Related pages 
Population growth
World Population Day
Malthusianism

Further reading

References

Other websites
Pan Earth (overpopulation website)
The story of B. The Great Forgetting (book outline with explanation by Daniel Clarence Quinn)
Population Institute of Canada

Demography
Population
Ecology
Environmental issues