Question

what exactly is an invasive species?

Answer 1

There are quite a few definitions out there and it often is determined by the question you are asking but generally it contains these two things:
a) It is non-native to the area it is occurring
b) It can/is causing harm to the environment or humans.

Answer 2

The first definition, the most used, applies to non-indigenous species, or "non-native", plants or animals that adversely affect the habitats and bioregions they invade economically, environmentally, and/or ecologically. They disrupt by dominating a region, wilderness areas, particular habitats, and/or wildland-urban interface land from loss of natural controls (i.e.: predators or herbivores). This includes non-native invasive plant species labeled as exotic pest plants and invasive exotics, in restoration parlance, growing in native plant communities.[1] It has been used in this sense by government organizations[2][3] as well as conservation groups such as the International Union for Conservation of Nature (IUCN) and the California Native Plant Society.[4] The European Union defines "Invasive Alien Species" as those that are, firstly, outside their natural distribution area, and secondly, threaten biological diversity.[5] It is also used by land managers, botanists, researchers, horticulturalists, conservationists, and the public for noxious weeds.[6] The kudzu vine (Pueraria lobata), Andean Pampas grass (Cortaderia jubata), and yellow starthistle (Centaurea solstitialis) are examples.