An "invasive species" is defined as a species that is non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health. (Executive Order 13112).
An invasive species grows/reproduces and spreads rapidly, establishes over large areas, and persists. Species that become invasive succeed due to favorable environmental conditions and lack of natural predators, competitors and diseases that normally regulate their populations.
This includes a wide variety of plants, insects and animals from exotic places. As invasive species spread and take over ecosystems, they decrease biodiversity by threatening the survival of native plants and animals. In fact, invasive species are a significant threat to almost half of the native U.S. species currently listed as federally endangered.
In addition to negatively impacting ecosystems, invasive species are also costly. It is very expensive to prevent, monitor and control the spread of invasives, not to mention the damage to crops, fisheries, forests, and other resources. Invasives cost the US $137 billion annually. Some of the most harmful species cost in excess of $100 million annually.
Sometimes you will see invasive species referred to as exotic, alien, or non-indigenous species. The problem with these names is that they only refer to the non-native part of the definition above. Many exotic or alien species do not cause harm to our economy, our environment, or our health. In fact, the vast majority of "introduced" species do not survive and only about 15% of those that do go on to become "invasive" or harmful.