A pest is any living thing, whether animal, plant, or fungus, that has a negative effect on humans. The term is particularly used for creatures that damage crops, livestock, and forestry or cause a nuisance to people, especially in their homes). Pests can be unwanted plants (weeds), fungi, nematodes, microbes (such as bacteria or viruses), insects, spiders, mites, birds, fish, rodents, or even deer. Labeling an organism as a pest is a subjective concept that varies with each individuals point of view. In general, pests are unwanted or undesirable because they reduce the availability, quality, or value of human resources such as food, feed, water, or space; injure humans, animals, crops, structures, and possessions; spread or cause disease; or interfere with human activities by causing annoyance, discomfort, or inconvenience.

It is important to note that not all organisms that humans consider pests are inherently pests, and many organisms may become pests. For example, over 97 percent of insects in and around the home are beneficial or innocent bystanders. Even critters we often think of as pests can play an important role in the ecosystem, such as food for fish and birds (mosquitoes and blackflies), eating garden pests (wasps), pollinating fruit trees (bees), and more.

In addition to the biological definition of pest, there is also a business concept called PEST analysis, which stands for political, economic, social, and technological factors that could affect a business now and in the future. The purpose of a PEST analysis is to help a companys management team better understand the market theyre operating in now and how they can prepare for potential shifts (for example, regulatory changes) .