An invasive species is an introduced species that harms its new environment. Invasive species adversely affect habitats and bioregions, causing ecological, environmental, and/or economic damage. Since the 20th century, invasive species have become serious economic, social, and environmental threats worldwide.

Invasion of long-established ecosystems by organisms is a natural phenomenon, but human-facilitated introductions have greatly increased the rate, scale, and geographic range of invasion. For millennia, humans have served as both accidental and deliberate dispersal agents, beginning with their earliest migrations, accelerating in the Age of Discovery, and accelerating again with the spread of international trade. Notable invasive plant species include the kudzu vine, giant hogweed (Heracleum mantegazzianum), Japanese knotweed (Reynoutria japonica), and yellow starthistle (Centaurea solstitialis). Notable invasive animals include European rabbits (Oryctolagus cuniculus), domestic cats (Felis catus), and carp (family Cyprinidae).

Invasive species are the subset of established non-native alien or naturalized species that are a threat to native species and biodiversity. The term "invasive" is poorly defined and often very subjective. Invasive species may be plants, animals, fungi, and microbes; some include native species that have invaded human habitats such as farms and landscapes. Some broaden the term to include indigenous or "native" species that have colonized natural areas. Some sources name Homo sapiens as an invasive species, but broad appreciation of human learning capacity and their behavioral potential and plasticity may argue against any such fixed categorization. The definition of "native" can also be controversial. For example, the ancestors of modern horses (Equus ferus) evolved in North America and radiated to Eurasia before becoming extinct in North America. Upon being introduced to North America in 1493 by Spanish conquistadors, it is debatable whether the feral horses were native or exotic to the continent of their evolutionary ancestors.

While invasive species can be studied within many subfields of biology, most research on invasive organisms has been in ecology and biogeography. Much of the work has been influenced by Charles Elton's 1958 book The Ecology of Invasion by Animals and Plants which creates a generalized picture of biological invasions. Studies remained sparse until the 1990s. This research, largely field observational studies, has disproportionately been concerned with terrestrial plants. The rapid growth of the field has driven a need to standardize the language used to describe invasive species and events. Despite this, little standard terminology exists. The field lacks any official designation but is commonly referred to as "invasion ecology" or more generally "invasion biology". This lack of standard terminology has arisen due to the interdisciplinary nature of the field, which borrows terms from disciplines such as agriculture, zoology, and pathology, as well as due to studies being performed in isolation.

In an attempt to avoid the ambiguous, subjective, and pejorative vocabulary that so often accompanies discussion of invasive species even in scientific papers, Colautti and MacIsaac proposed a new nomenclature system based on biogeography rather than on taxa. By discarding taxonomy, human health, and economic factors, this model focused only on ecological factors. The model evaluated individual populations rather than entire species. It classified each population based on its success in that environment. This model applied equally to indigenous and to introduced species, and did not automatically categorize successful introductions as harmful.

The USDA's National Invasive Species Information Center defines invasive species very narrowly. According to Executive Order 13112, "'Invasive species' means an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health."

Typically, an introduced species must survive at low population densities before it becomes invasive in a new location. At low population densities, it can be difficult for the introduced species to reproduce and maintain itself in a new location, so a species might need to reach a location multiple times before it becomes established. Repeated patterns of human movement, such as ships sailing to and from ports or cars driving up and down highways, offer repeated opportunities for establishment (referred to as a high propagule pressure).

In ecosystems, the availability of resources determines the impact of additional species on the ecosystem. Stable ecosystems have a resource equilibrium, which can be changed fundamentally by the arrival of invasive species. When changes such as a forest fire occur, normal ecological succession favors native grasses and forbs. An introduced species that can spread faster than natives can outcompete native species for food, squeezing the natives out. Nitrogen and phosphorus are often the limiting factors in these situations. Every species occupies an ecological niche in its native ecosystem; some species fill large and varied roles, while others are highly specialized. Invading species may occupy unused niches, or create new ones. For example, edge effects describe what happens when part of an ecosystem is disturbed, as in when land is cleared for agriculture. The boundary between the remaining undisturbed habitat and the newly cleared land itself forms a distinct new habitat, creating new winners and losers, and potentially hosting species that would not otherwise thrive outside the boundary habitat.