Naturalisation (or naturalization) is the ecological phenomenon through which a species, taxon, or population of exotic (as opposed to native) origin integrates into a given ecosystem, becoming capable of reproducing and growing in it, and proceeds to disseminate spontaneously. In some instances, the presence of a species in a given ecosystem is so ancient that it cannot be presupposed whether it is native or introduced.

Generally, any introduced species may (in the wild) either go extinct or naturalise in its new environment.

Some populations do not sustain themselves reproductively, but exist because of continued influx from elsewhere. Such a non-sustaining population, or the individuals within it, are said to be adventive. Cultivated plants, sometimes called nativars, are a major source of adventive populations.

In botany, naturalisation is the situation in which an exogenous plant reproduces and disperses on its own in a new environment. For example, northern white cedar is naturalised in the United Kingdom, where it reproduces on its own, while it is not in France, where human intervention via cuttings or seeds are essential for its dissemination.^{[citation needed]}

Two categories of naturalisation are defined from two distinct parameters: one, archaeonaturalised, refers to introduction before a given time (introduced over a hundred years ago), while the second, amphinaturalised or eurynaturalised, implies a notion of spatial extension (taxon assimilated indigenous and present over a vast space, opposed to stenonaturalised).^{[clarification needed][citation needed]}

The degrees of naturalisation are defined in relation to the status of nativity or introduction of taxons or species:

Animal naturalisation is mainly carried out through breeding and by commensalism following human migrations.

The concerned species are thus: