habitat Place or type of place where an organism or population of organisms lives. Compare ecological niche.

habitat fragmentation Breakup of a habitat into smaller pieces, usually as a result of human activities.

half-life Time needed for one-half of the nuclei in a radioisotope to emit its radiation. Each radioisotope has a characteristic half-life, which may range from a few millionths of a second to several billion years. See radioisotope.

hazard Something that can cause injury, disease, economic loss, or environmentaldamage. See also risk.

hazardous chemical Chemical that can cause harm because it (1) is flammable or explosive, (2) can irritate or damage the skin or lungs (such as strong acidic or alkaline substances), or (3) can cause allergic reactions of the immune system (allergens). See also toxic chemical.

hazardous waste Any solid, liquid, or containerized gas that (1) can catch fire easily, (2) is corrosive to skin tissue or metals, (3) is unstable and can explode or release toxic fumes, or (4) has harmful concentrations of one or more toxic materials that can leach out. See also toxic waste.

Heat Total kinetic energy of all the randomly moving atoms, ions, or molecules within a given substance, excluding the overall motion of the whole object. Heat always flows spontaneously from a hot sample of matter to a colder sample of matter. This is one way to state the second law of  hermodynamics. Compare temperature.

herbicide Chemical that kills a plant or inhibits its growth.

herbivore Plant-eating organism. Examples are deer, sheep, grasshoppers, and zooplankton. Compare carnivore, omnivore.

high Air mass with a high pressure. Compare low.

high-quality energy Energy that is concentrated and has great ability to perform useful work. Examples are high-temperature heat and the energy in electricity, coal, oil, gasoline, sunlight, and nuclei of uranium-235. Compare low-quality energy.

high-quality matter Matter that is concentrated and contains a high concentration of a useful resource. Compare low-quality matter.

high-throughput economy Situation in most advanced industrialized countries, in which ever-increasing economic growth is sustained by maximizing the rate at which matter and energy resources are used, with little emphasis on pollution prevention, recycling, reuse, reduction of unnecessary waste, and other forms of resource conservation. Also used high-waste society and throwaway society. Compare low-throughput economy, matter-recycling economy.

homeostasis Maintenance of favorable internal conditions in a system despite fluctuations in external conditions. See constancy, inertia, resilience.

host Plant or animal on which a parasite feeds.

human resources Physical and mental talents of people used to produce, distribute, and sell an economic good. Also human capital. Compare financial resources, manufactured resources, natural resources.

Humus Slightly soluble residue of undigested or partially decomposed organic material in topsoil. This material helps retain water and water-soluble nutrients, which can be taken up by plant roots.

hunter-gatherers People who get their food by gathering edible wild plants and other materials and by hunting wild animals and fish. Compare agricultural revolution, environmental revolution, industrial revolution, information and globalization revolution.

hydrocarbon Organic compound of hydrogen and carbon atoms. The simplest hydrocarbon is methane (CH4), the major component of natural gas.

hydroelectric power plant Structure in which the energy of falling or flowing water spins a turbine generator to produce electricity.

hydrologic cycle Biogeochemical cycle that collects, purifies, and distributes the earth's fixed supply of water from the environment to living organisms and then back to the environment. Also water cycle.

hydropower Electrical energy produced by falling or flowing water. See hydroelectric power plant.

Hydrosphere The earth's (1) liquid water (oceans, lakes, other bodies of surface water, and underground water), (2) frozen water (polar ice caps, floating ice caps, and ice in soil, known as permafrost), and (3) small amounts of water vapor in the atmosphere. See also hydrologic cycle.