Invasive species, also called invasive exotics or simply exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions.

Because of the variability of its definition, and because definitions are often from a socioeconomic perspective, the phrase invasive species is often criticized as an imprecise term for the scientific field of ecology. This article concerns the first two definitions; for the third, see Introduced species.

Scientists include species- and ecosystem factors among the mechanisms, that when combined establish invasiveness in a newly introduced species.

While all species compete to survive, invasive species appear to have specific traits or specific combinations of traits that allow them to outcompete native species. In some cases the competition is about rates of growth and reproduction. In other cases species interact with each other more directly.

Researchers disagree about the usefulness of traits as invasiveness markers. One study found that of a list of invasive and noninvasive species, 86% of the invasive species could be identified from the traits alone. Another study found invasive species tended only to have a small subset of the presumed traits, and that many such traits were found in noninvasive species, requiring other explanations. Common invasive species traits include:

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 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 (also known as a high propagule pressure).