A "refugee" is defined as a person who has crossed an international border "due to well-founded fear of being persecuted on the grounds of race, religion, nationality, membership of a particular social group, or political opinion" (UNHCR 2019, 1951 Convention on to the Refugee Statute)

The Guiding Principles on Internal Displacement define internally displaced persons (IDPs) as "persons or groups of persons who have been forced or forced to flee or leave their homes, as a result of armed conflict, situations of widespread violence, violation of human rights or natural or man-made disasters that have not crossed an international border. " Internally displaced persons remain citizens or habitual residents of their country and are entitled to protection and assistance only on that basis.

A crucial requirement to be considered a "refugee" is to cross an international border. People forcibly displaced from their homes who cannot or choose not to cross a border are therefore not considered refugees, even if they share many of the same circumstances and challenges as those who do. Unlike refugees, these internally displaced persons do not have a special status in international law with rights specific to their situation. UNHCR's Guiding Principles on Internal Displacement include the effects of development projects, natural disasters and man-made disasters among the drivers of forced migration within borders. However, official refugee status has not been granted for environmental reasons or linked to development-induced displacement, but focuses on people affected by war and conflict.

The definition of who is considered an "environmental refugee" is highly controversial. Should this term define only those people who were displaced due to natural disasters or also those affected by human degradation of the environment (such as mining and oil extraction), development projects and climate change? The term "environmental refugee" is often used in the media and other discussions. However, this phrase can cause confusion, because this category does not exist in international law. Therefore, people who are environmental refugees do not receive any international protection or aid. According to UNHCR's definition, there may be situations where the “refugee” criteria are applied to environmental refugees. For example, if drought-related famine is linked to situations of armed conflict and violence, a concept known as "link dynamics".