Invasive species is a phrase with several definitions.
The first definition expresses the phrase in terms of non-indigenous species (e.g. plants or animals) that adversely effect the habitats they invade economically, environmentally or ecologically.
It has been used in this sense by government organizations as well as conservation groups such as the IUCN.
The second definition broadens the boundaries to include both native and non-native species that heavily colonize a particular habitat.
The third definition is an expansion of the first and defines an invasive species as a widespread non-indigenous species.
This last definition is arguably too broad as not all non-indigenous species necessarily have an adverse effect on their adopted environment.
An example of this broader use would include the claim that the common goldfish (Carassius auratus) is invasive.
True, it is common outside of its range globally but it almost never appears in harmful densities.
Due to the ambiguity of its definition, the phrase - invasive species - is often criticized as an imprecise term within the field of ecology.
Scientists propose several mechanisms to explain invasive species, including: species-based mechanisms and ecosystem-based mechanisms, most likely, it is a combination of several mechanisms that cause an invasive situation to occur since most introduced plants and animals do not become invasive.