A bioregion is a geographical area defined not by administrative boundaries, but by distinct characteristics such as plant and animal species, ecological systems, soils and landforms, human settlements, and topographic features such as drainage basins (also referred to as "watersheds"). A bioregion can be on land or at sea. The idea of bioregions was adopted and popularized in the mid-1970s by a school of philosophy called bioregionalism, which includes the concept that human culture can influence bioregional definitions due to its effect on non-cultural factors. Bioregions are part of a nested series of ecological scales, generally starting with local watersheds, growing into larger river systems, then Level III or IV ecoregions (or regional ecosystems), bioregions, then biogeographical realm, followed by the continental-scale and ultimately the biosphere.

Within the life sciences, there are numerous methods used to define the physical limits of a bioregion based on the spatial extent of mapped ecological phenomena—from species distributions and hydrological systems (i.e. Watersheds) to topographic features (e.g. landforms) and climate zones (e.g. Köppen classification). Bioregions also provide an effective framework in the field of Environmental history, which seeks to use "river systems, ecozones, or mountain ranges as the basis for understanding the place of human history within a clearly delineated environmental context". A bioregion can also have a distinct cultural identity defined, for example, by Indigenous Peoples whose historical, mythological and biocultural connections to their lands and waters shape an understanding of place and territorial extent. Within the context of bioregionalism, bioregions can be socially constructed by modern-day communities for the purposes of better understanding a place "with the aim to live in that place sustainably and respectfully."

Bioregions have practical applications in the study of biology, biocultural anthropology, biogeography, biodiversity, bioeconomics, bioregionalism, bioregional mapping, community health, ecology, environmental history, environmental science, foodsheds, geography, natural resource management, urban Ecology, and urban planning. References to the term "bioregion" in scholarly literature have grown exponentially since the introduction of the term—from a single research paper in 1971 to approximately 65,000 journal articles and books published to date. Governments and multilateral institutions have utilized bioregions in mapping Ecosystem Services and tracking progress towards conservation objectives, such as ecosystem representation.

The first confirmed use of the term "bioregion" in academic literature was by E. Jarowski in 1971, a marine biologist studying the blue crab populations of Louisiana. The author used the term sensu stricto to refer to a "biological region"—the area within which a crab can be provided with all the resources needed throughout its entire life cycle. The term was quickly adopted by other biologists, but eventually took on a broader set of definitions to encompass a range of macro-ecological phenomena.

The term bioregion as it relates to bioregionalism is credited to Allen Van Newkirk, a Canadian poet and biogeographer. In this field, the idea of "bioregion" likely goes back much earlier than published material suggests, being floated in early published small press zines by Newkirk, as well as in conversational dialogue. This can be exemplified by the fact that Newkirk had met Peter Berg (another early scholar on bioregionalism) in San Francisco in 1969 and again in Nova Scotia in 1971 where he shared the idea with Berg. He would go on to found the Institute for Bioregional Research and issue a series of short papers using the term bioregion as early as 1970. Peter Berg, who would go on to found the Planet Drum foundation, and become a leading proponent of "bioregions" learned of the term in 1971 while Judy Goldhaft and Peter Berg were staying with Allen Van Newkirk, before Berg attended the first United Nations Conference on the Human Environment in Stockholm during June 1972. The Planet Drum Foundation published their first Bioregional Bundle in that year, that also included a definition of a bioregion. Helping refine this definition, Author Kirkpatrick Sale wrote in 1974 that "a bioregion is a part of the earth's surface whose rough boundaries are determined by natural rather than human dictates, distinguishable from other areas by attributes of flora, fauna, water, climate, soils and landforms, and human settlements and cultures those attributes give rise to.

Several other marine biology papers picked up the term, and in 1974 the International Union for Conservation of Nature (IUCN) published its first global-scale biogeographical map entitled "Biotic Provinces of the World". However, in their 1977 article "Reinhabiting California", director of the IUCN and founder of the Man and Biosphere project Raymond Dasmann and Peter Berg pushed back against these global bodies that were attempting to use the term bioregion in a strictly ecological sense, which separated humans from the ecosystems they lived in, specifically naming that Biotic Provinces of the World Map, was not a map of bioregions.

"Reinhabitation involves developing a bioregional identity, something most North Americans have lost or have never possessed. We define bioregion in a sense different from the biotic provinces of Raymond Dasmann (1973) or the biogeographical province of Miklos Udvardy. The term refers both to geographical terrain and a terrain of consciousness—to a place and the ideas that have developed about how to live in that place. Within a bioregion, the conditions that influence life are similar, and these, in turn, have influenced human occupancy."

This article defined bioregions as distinct from biogeographical and biotic provinces that ecologists and geographers had been developing by adding a human and cultural lens to the strictly ecological idea.