Environmental migrants are people who are forced to leave their home of residency due to sudden or long-term changes to their local or regional environment. These changes compromise their well-being or livelihood, and include increased drought, desertification, sea level rise, and disruption of seasonal weather patterns (such as monsoons). Though there is no uniform, clear-cut definition of environmental migration, the idea is gaining attention as policy-makers and environmental and social scientists attempt to conceptualize the potential social effects of climate change and other environmental degradation. Environmental migration also reflects broader global inequalities, as marginalized and racialized communities often experience the most severe environmental degradation while having the least resources and political power to adapt or relocate. Environmental migrants originate from a variety of different locations, including Small Island Developing States.

"Environmental migrant" and "climate migrant" (or "climate refugee") are used somewhat interchangeably with a range of similar terms, such as ecological refugee, environmental refugee, forced environmental migrant, environmentally motivated migrant, environmentally displaced person (EDP), disaster refugee, environmental displacee, eco-refugee, ecologically displaced person, or environmental-refugee-to-be (ERTB). The distinctions between these terms remain contested.

The term "environmental refugee" was first proposed by Lester Brown in 1976. The International Organization for Migration (IOM) proposes the following definition for environmental migrants:

"Environmental migrants are persons or groups of persons who, for compelling reasons of sudden or progressive changes in the environment that adversely affect their lives or living conditions, are obliged to leave their habitual homes, or choose to do so, either temporarily or permanently, and who move either within their country or abroad."

IN 1985, UNEP (United Nations Environment Programme) researcher Essam El-Hinnawi defined environmental refugees as:

"those people who have been forced to leave their traditional habitat, temporarily or permanently, because of a marked environmental disruption (natural and/or triggered by people) that jeopardized their existence and/or seriously affected the quality of their life [sic]. By 'environmental disruption' in this definition is meant any physical, chemical, and/or biological changes in the ecosystem (or resource base) that render it, temporarily or permanently, unsuitable to support human life."

Climate migrants are a subset of environmental migrants who were forced to flee "due to sudden or gradual alterations in the natural environment related to at least one of three impacts of climate change: sea-level rise, extreme weather events, and drought and water scarcity."

Critics of these definitions argue that they overlook the structural and political drivers of displacement, such as colonial legacies, racialized land use policies, and unequal resource extraction. Scholars in environmental justice studies emphasize that these factors shape who becomes displaced, making environmental migration a social and political issue as much as an ecological one.