Invasive plants are generally not native to an area and have few natural pests to keep them in control. When the habitat is favorable they can multiply out of control and replace or damage natural plants and habitats to the detriment of all of the other plants and animals that share it. Controlling these invasions is difficult and often impossible.
Invasivespecies.gov defines an invasive species as follows:
An “invasive species” is defined as a species that is 1) non-native (or alien) to the ecosystem under consideration and 2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health. (Executive Order 13112).
Invasive species can be plants, animals, and other organisms (e.g., microbes). Human actions are the primary means of invasive species introductions.
They also define the following related terms:
Alien species means, with respect to a particular ecosystem, any species, including its seeds, eggs, spores, or other biological material capable of propagating that species, that is not native to that ecosystem.
Control means, as appropriate, eradicating, suppressing, reducing, or managing invasive species populations, preventing spread of invasive species from areas where they are present, and taking steps such as restoration of native species and habitats to reduce the effects of invasive species and to prevent further invasions.
Ecosystem means the complex of a community of organisms and its environment.
Introduction means the intentional or unintentional escape, release, dissemination, or placement of a species into an ecosystem as a result of human activity.
Invasive species means an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.
Native species means, with respect to a particular ecosystem, a species that, other than as a result of an introduction, historically occurred or currently occurs in that ecosystem.
Species means a group of organisms all of which have a high degree of physical and genetic similarity, generally interbreed only among themselves, and show persistent differences from members of allied groups of organisms.