is a plant or animal that is out of place
in the the environmental hierarchy
The most obvious example is a weed--a plant that out-competes those around it, causing them to die. Broad leaf plants, specifically, in lawns will out-compete grass for sunlight.
More generally, pests appear in areas cleared of their ecological diversity--what is now called its bio-diversity--and have no predators to keep them in check. All species seek to proliferate.
A lawn is a perfect example--a monoculture, one species of something living along, does not exist in nature. So it will be invaded by broad-leafed weeds.
Similarly, in agriculture, vast fields of one crop, often genetically altered to be the same plant--again no bio-diversity--is the perfect arena for some pest to invade and proliferate without control. Hence the great need for pesticides.
The technology of agriculture generates the need for the technology of pesticides, the techology of pesticides generates the need for the technology of genetic engineering of pest-resistant plants, and the technology of pest-resistant plant generates the need for...what new technology?
This is why I argue that technology is not the remedy for problems caused by technology.