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H

Haber process - The chemical process used to make ammonia.
Habitat - The physical area in which an organism lives.
Hadal zone - The floor of the deepest ocean trenches down to 10,000 meters.
Hadrons - Massive subatomic particles composed of quarks.
Half-life - The length of time it takes for one-half of the atoms of a radioactive nuclide to disintegrate.
Half-reaction - One half of an oxidation-reduction reaction.
Hall-Heroult process - The chemical process used to extract aluminum from bauxite ore.
Hallucinogens - Drugs that disrupt nerve actions causing altered senses.
Halogens - Elements in Group 17 on the periodic table.
Halophiles - "Salt-loving" bacteria living in environments with a very high salt concentration that would kill other bacteria.
Hangman - Review Game
Haploid - Refers to a cell containing only one chromosome of a pair.
Hard water - Water with a high mineral content, usually calcium.
Hardy-Weinberg Principle - A population will remain in genetic equilibrium if, and only if, a certain set of conditions are met.
Haversian canals - Pores in bones through which blood vessels can pass.
Head - A rounded or flat-topped cluster of sessil flowers.
Heart - The primary organ of the circulatory system.
Heartwood - The dark center of most trees, consisting of clogged xylem cells.
Heat - The motion of the particles of matter.
Heat engine - A machine that burns some type of fuel to produce heat, then convert the heat energy into mechanical energy.
Heat of fusion - The amount of heat needed to change one gram of a substance from the solid phase to the liquid phase.
Heat of vaporization - The amount of heat needed to change one gram of a substance from the liquid phase to the gas phase.
Heat transfer - Heat flowing from an area of high heat to an area low in heat.
Helix - A spiral; a twisted spiral called a double helix is the basic shape of the DNA molecule.
Hemorrhagin - A chemical in snake venom that causes lysis (breaking down) of cells in the capillaries causing hemorrhaging in skin and mucous membranes.
Hemotoxin - Venom proteins that destroy red blood cells and disrupt blood clotting.
Herbaceous - Plants whose stems have very little sclerenchyma tissue, making them soft.
Herbarium - A humidity and temperature controlled container or room that protects a scientific collection of plants from insect infestation.
Herbivores - Consumers that eat only plants.
Heredity - The passing of traits from one generation to the next.
Hermaphroditic - Being able to produce both sperm and egg cells.
Hertz - The units of wave frequency.
Hesperidium - A fleshy fruit with a tough rind, numerous carpels separable as 'sections'.
Hess's Law - The Enthalpy change for a reaction is the sum of the enthalpy changes for a series of reactions that add up to the overall reaction.
Heterodont - Having several types of teeth.
Heterogeneous - Not identical throughout.
Heterotrophic - Organisms that are not able to produce their own food.
Heterozygous - Refers to things that are different.
Hibernation - A period of inactivity during the winter.
Hilum - The funicular scar on a seed coat.
HIV - Human immunodeficiency virus, the virus responsible for AIDS.
Hologram - A three-dimensional image produced with laser light.
Holotype - The original specimen designated by the author in a scientific collection.
Homeostasis - A stable level of internal conditions in living things.
Homogeneous - Identical propertics throughout.
Homologous chromosomes - The two members of each pair of chromosomes in regular body cells.
Homozygous - Refers to things that are the same.
Horns - Extensions from the skull of all species of bovids.
Horsepower - A unit of power equal to 745.56 Watts.
Hotwater bath - A lab technique used to heat something slowly and evenly.
Human genome - All the genes found on human chromosomes.
Humus - The organic matter in the soil.
Hund's Rule - Electrons will enter empty orbitals of equal energy, when they are available.
Hybrid - An organism produced by crossing individuals with different traits.
Hybrid vigor - Hybrid organisms are more successful than those that are produced from a very limited gene pool.
Hydrated crystal - A crystal in which water molecules have become chemically bonded to the ions.
Hydraulics - The study of the pressure exerted by fluids.
Hydrocarbon - A substance composed of only hydrogen and carbon.
Hydrogen bond - A covalent bond between hydrogen and a highly electronegative atom.
Hydrolysis - The reaction of a salt with water to form an acid or base solution.
Hydronium ion - Formed by a hydrogen ion and a water molecule, H₃O ⁺.
Hygroscopic - A substance that will capture water molecules out of the air.
Hygroscopic water - Water that is tightly bound to the soil particles and unavailable to plant roots.
Hydrotropism - A plant growth response to water.
Hypanthium - The cup of the receptacle from which all flower parts arise.
Hypha - The vegetative filament of a fungus.
Hypocotyl - The part of a plant embryo that will form the stem of the new plant.
Hypogynous - A flower with the ovary situated above the calyx with no floral cup around it.

Ideal gas - A theoretical gas that has mass but no volume.
Identical twins - Formed from one egg cell fertilized by one sperm cell.
Immigration - The movement of individuals into a population.
Imperfect flower - A flower with either stamens or pistils, but not both.
Implantation - The blastocyst attaches to the uterine lining.
Incandescent light - Electricity passes through a wire filament making it glow.
Incisors - Knife-edged teeth for biting and cutting.
Inclined plane - A slanted surface used to raise an object.
Incomplete flower - A flower missing one of more of the four basic parts.
Indehiscent - A dry fruit that does not open when ripe.
Independent variable - The variable in an experiment controlled by the experimentor; the variable plotted on the X axis of a graph; the variable listed on the left side of a data table.
Index of refraction - A comparison of the speed of light in air with the speed of light in another material.
Indicators - Compounds that show a color change in an acid or base.
Induction - A neutral object acquires a charge from a charged object close by without contact being made.
Inertia - The resistance of an object to changes in motion.
Inflorescence - An arrangement of several flowers on a single stalk.
Inhalants - Toxic substances that are inhaled for the rush.
Inhibitor - A substance that ties up a reactant in a complex, so that it will not react.
Inner ear - Contains the cochlea, and is responsible for hearing and balance.
Insect - An arthropod with three body parts and six legs.
Insertion - The point of movable attachment of a skeletal muscle.
Insoluble - A substance does not dissolve in a solvent.
Insulators - Materials that do not easily allow the flow of heat or electric current.
Integrated circuit - Combines diodes and transistors on a thin slice of silicon crystal.
Integument - The body covering, skin, of an organism.
Interference - The interaction of two or more waves to produce a single new wave.
Interferon - A protein produced by cells when exposed to a virus.
Intermembranous bone - Flat bone that develops between two layers of fibrous connective tissue without the cartilage stage and has no system of Haversian canals.
Internal combustion engine - A heat engine in which fuel is burned inside the engine.
Internal respiration - The exchange of gases between blood and body cells.
Internodes - The intervals between nodes on a plant stem.
Interphase - The period of normal growth and activity in the cell cycle.
Interpolate - Predicting data between two measured points on the graph.
Intertidal zone - Shore areas that are alternately covered and uncoverd by the tide.
Invasive species - A species that moves into an area and pushes native species out.
Invertebrate - An animal with no vertebral column.
Irregular flower - A flower with petals and sepals that are dissimilar, the flower is not symmetrical.
Ion - An atom or group of atoms with a charge.
Ionic bond - Electrons are transferred between atoms to form a compound.
Ionization - The process of removing an electron from an atom to form an ion.
Ionization energy - The energy needed to remove an electron from an atom.
Iris - The muscular, colored area around the eye's pupil that controls the amount of light entering the eye.
Isobaric - Taking place at a constant pressure.
Isothermal - Taking place at a constant temperature.
Isotopes - Atoms of the same element with different amounts of neutrons.
Isotype - Any specimen from the original collection used to duplicate the holotype.

Joint - A place where two bones meet.
Joule - A unit of work, Newton-meter.

Kaon - A subatomic particle composed of a quark and an antiquark.
Karyotype - An arrangement of the pairs of chromosomes of an organism, from largest to smallest.
K-capture - An atomic nucleus captures an electron from its own innermost energy level.
Kelvin - The temperature scale used in all gas law calculations.
Ketones - Organic compounds containing the carbonyl group.
Kilo - Metric system unit for 1000.
Kinetic energy - Energy an object has because of its motion.
Kinetic Theory - Explains the effects of temperature and pressure on matter.
Kingdom - The largest classification grouping of organisms.
Krebs Cycle - An aerobic cellular process that produces 2 ATP molecules, 10 carrier molecules, and CO₂ from each glucose molecule.
K-selected species - A species with more or less stable populations adapted to exist at or near carrying capacity in relatively stable habitats.

Lambda Convention - A formal system used to determine the chemical formulas of organic compounds.
Larva - An early stage of metamorphosis that looks nothing like the adult.
Laser - Coherent light of only one frequency.
Latitude - Describes north or south of the equator.
Lateral line - A row of scales with sensory depressions down each side of the fish.
Lateral meristem - Located around the outer edge of stems and roots, this meristem produces growth in circumference known as secondary growth.
Law of Conservation of Matter and Energy - The total matter and energy in the universe is always conserved.
Law of Superposition - Successive layers of rock or soil have been deposited on top of one another by wind or water.
Le Chatelier‘s Principle - If stress is applied to a system at equilibrium, the system will tend to readjust so that the stress is reduced.
Left-hand Rules - Two rules related to current flow and poles in electromagnets.
Legume - A dry fruit with 1 carpel that splits along both sides when ripe.
Lens - A curved, transparent object that bends light rays.
Lentic ecosystem - A freshwater ecosystem composed of standing water like lakes, ponds, marshes, and swamps.
Leptons - Lightweight elementary particles not composed of quarks.
Leukemia - The uncontrolled production of white blood cells.
Leukocytes - White blood cells.
Lever - A bar which can move freely around a fixed position, or fulcrum.
Lewis Theory - One of the three common acid/base theories.
Lichen - A symbiotic association between a fungus and a photosynthetic partner, usually a cyanobacterium or green alga.
Ligament - Tough, flexible bands of connective tissue holding bones together at joints.
Ligand - A negative ion or polar molecule bonded to the central ion in a complex.
Light - A form of energy produced by excited atoms.
Light microscope - Produces magnification by bending light rays with lenses.
Limbic system - A combination of the thalamus and hypothalamus that work together in the brain to regulate emotions.
Limiting reactant - The reactant that is completely consumed in a chemical reaction.
Lipids - Large organic molecules with a higher ratio of carbon and hydrogen atoms to oxygen atoms than carbohydrates; fats.
Liquid - A substance whose particles have enough kinetic energy to stretch the intermolecular forces of attraction.
Liquid crystal - A crystal without a sharp melting point temperature.
Littoral zone - The highly variable intertidal zone of the ocean.
Litmus - A common paper acid/base indicator.
Loam - Soil containing a combination of sand, silt, and clay.
Locules - The chambers of a plant ovary.
Long-day plants - Plants that flower only when exposed to day lengths longer than their critical length.
Longitude - Describes east or west of the prime meridian.
Longitudinal wave - The medium particles move parallel to the direction of wave travel.
Lotic ecosystem - A freshwater ecosystem composed of flowing water like rivers and streams.
Lungfish - A small group of fish that have lungs as well as gills.
Lytic cycle - The reproductive cycle of a virulent virus.
Lymph - An intercellular fluid found in body tissues.
Lymphoma - Cancer of the lymph cells.
Lysogenic cycle - The reproductive cycle of a temperate virus.
Lysosome - Digestive centers of a cell.