Overpopulation is the condition of any organism's numbers exceeding the carrying capacity of its ecological niche. In common parlance, the term usually refers to the relationship between the human population and its environment, Earth.
In the context of human societies, however, overpopulation occurs when the population density is so great as to actually cause an impaired quality of life, serious environmental degradation, or long-term shortages of essential goods and services. This is the definition used by popular dictionaries such Merriam-Webster. If this causal relationship cannot be proven to exist, overpopulation cannot be said to occur, as such an imbalance may actually be caused by any number of other factors such as bad governance, war, corruption or endemic poverty.