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D

Dalton's Law - The total pressure in a container is the sum of the partial pressures of all the gases in the container.
Dalton's Theory - The first atomic theory, in 1803.
Daughter cell - The new cell formed when a cell divides.
Day-neutral plants - Plants whose flowering is not affected by the length of days and nights.
Decapod - An animal with ten feet, five pairs of legs.
Deceleration - A term commonly used to mean a decrease in speed; negative acceleration.
Decomposers - Organisms that break down dead organic matter into smaller organic materials and ultimately into inorganic matter.
Decomposition reaction - One complex substance breaks down into two or more simple substanes.
Dehiscent - A dry fruit that opens when ripe.
Dehydration - A term referring to a multitude of cellular problems that develop as the amount of water in the body begins to get too low.
Deliquescent - A solid so strongly hygroscopic that it will dissolve in air.
Dendrite - A finger-like projection from the neuron body that receives impulses.
Dendrochronology - The study of climate history using tree rings.
Denitrification - Anaerobic bacteria break down nitrates, releasing N₂ into the atmosphere.
Density - Mass per unit of volume.
Dental formula - An expression of the number of each type of tooth on one side of the upper jaw over the number of each type of tooth on one side of the lower jaw.
Dentine - The porous, bone-like foundation of a tooth.
Dependent variable - The variable in an experiment that changes with the independent variable; the variable plotted on the Y axis of a graph; the variable listed on the right side of a data table.
Depressants - Drugs that slow down nerve action, heart rate, and lower blood pressure.
Derived units - Combinations of basic SI units.
Desiccant - An anhydrous substance used to remove water from other substances.
Diadelphous - A flower whose stamens are attached by their filaments into two groups.
Diaphragm - The breathing muscle located below the ribs in mammals.
Diatomic molecules - Seven elements existing in pure form in nature only as two atoms covalently bonded.
Dichasium - A peduncle bearing a terminal flower and a pair of branches producing lateral flowers.
Dichotomous key - A device to use to identify organisms.
Dicots - Plants with two seed leaves.
Diffraction - The bending of waves around the edge of a barrier.
Diffusion - The movement of particles from an area of high concentration to an area of low concentration.
Digestion - The two-part process that changes food into a form usable by body cells.
Dihybrid cross - A genetic cross involving two traits.
Dilute - A solution with a small amount of solute.
Diode - The basic, one-way gate for electrons in electronic devices.
Dipeptide - Two amino acids bonded together.
Diphyodont - Having two sets of teeth.
Diploid - Refers to a cell that contains both chromosomes of a homologous pair.
Dipole - A polar molecule.
Dipole-induced dipole forces - A dipole transforms a nonpolar molecule into a dipole and an attraction occurs.
Direct current - DC electricity whose current flows in one direction only.
Disaccharide - A double sugar composed of two simple sugars.
Dislocation - A fault in which unit cells of a crystal are joined improperly.
Dispersal unit - Refers to any detached plant part serving as a vehicle for seed dispersal.
Dispersion forces - The attraction of two nonpolar molecules.
Dissociation - The separation of ions from a compound during solution.
Distillation - Removing dissolved substances from a liquid or separating a mixture of liquids that have different boiling points.
Divergent evolution - Two or more similar species becoming more and more dissimilar because of environmental adaptations.
DNA - Deoxyribonucleic acid stores and transmits the genetic information in cells.
Doping - Adding impurities to a semiconductor to increase its conductivity.
Doppler effect - A change in the frequency and pitch of a sound due to the motion of either the sound source or the observer.
Dorsal - Refers to the top surface of an organism.
Double displacement reaction - Different atoms in two different compounds replace each other.
Double helix - The spiral shape of a DNA molecule.
Drug - Any substance taken into the body that alters normal body processes.
Drupe - A fleshy fruit composed of one carpel with a single seed, fleshy outer and inner stony layers.
Dry cell - An electrochemical cell in which all reactants are solids.
Dry gas - A gas with no water vapor.
Dry fruit - A fruit with papery or leathery walls.
Dura mater - The outer layer of the three protective tissues of the brain, consisting of connective tissues, blood vessels, and nerves.
Dysphotic zone - Middle layers of ocean water consisting of twilight that does not support alga growth.

Eardrum - A tightly stretched membrane in the outer ear that vibrates as the sound waves strike it.
E. Coli - Common bacteria living in our intestinal tract.
Ecological pyramid - A chart representing trophic level numbers within an ecosystem.
Ecology - The study of the interactions between organisms and the living and nonliving components of their environment.
Ecosystem - A functional environmental unit, consisting of a biotic community and the abiotic factors on which the organisms depend.
Ectocarp - The outermost layer of the pericarp, exocarp.
Ectoderm - The outer layer of cells of a gastrula that becomes the body covering.
Ectoparasite - A parasite living on the external surface of its host.
Ectothermic - Body temperature is determined by the environment; cold-blooded.
Effectors - Nerve cells that receive information from the central nervous system and transmit to muscles and glands.
Efficiency - A comparison of work output to work input.
Effort force - The force applied to a machine.
Elastic collision - A collision in which the momentum of the colliding particles is exactly the same after the collision as before.
Electricity - The energy associated with charged particles.
Electric circuit - A complete, closed path for an electric current.
Electric field - An area around a charged particle in which forces of attraction or repulsion are noticeable.
Electric motor - A device that converts electrical energy into mechanical energy.
Electrochemical cell - A device that changes chemical energy into electrical energy.
Electrolysis - The process by which an electric current produces a chemical change.
Electrolytes - Substances that produce ions in solution.
Electrolytic conduction - Movement of ions in solution from an area of high energy toward an area of low energy, producing a current.
Electromagnet - A temporary magnet produced by direct current running through a coil of wire wrapped around a soft iron core.
Electromagnetic force - The force of attraction or repulsion between charged particles.
Electromagnetic induction - When a conducting wire cuts across magnetic lines of force, producing an electric current.
Electromagnetic spectrum - An arrangement of all electromagnetic waves by decreasing wavelength, and therefore increasing frequency.
Electromotive force - Another term for volts.
Electron - A subatomic particle with a negative charge normally found in energy levels outside the nucleus of an atom; discovered by J. J. Thomson in 1897.
Electronegativity - The refative strength of the pull of an atom for electrons .
Electron affinity - The tendency of an atom to attract electrons.
Electron cloud repulsion - The natural repulsion of the electrons of two different atoms that keeps most atoms from reacting.
Electron configuration - A way to indicate the location of every electron in an atom.
Electron-dot diagram - A way of drawing the outer energy level electrons of an atom.
Electronic balance - A scale for measuring the mass of matter.
Electronics - The study of the release, behavior, and effects of electrons in useful devices.
Electron microscope - Produces magnification by bending streams of electrons with magnets.
Electron volt - A measure of energy equal to the energy gained by a single electron when it is accelerated across an electric potential difference of 1 volt: 1 eV = 1.602 X 10 ^-19 joules.
Elementary crystal - Any crystal bonded by normal electrostatic (ionic) forces.
Elements - The simplest pure substances, composed of the same type of atoms.
Embryo - The mass of cells formed by the divisions of a zygote.
Emigration - The movement of individuals out of a population.
Empirical formula - The simplest formula that represents the whole number ratio between the elements in a compound.
Enamel - A hard, smooth covering to protect the crown of a tooth.
Endochondral bone - Dense bone that forms by replacing cartilage and has a system of Haversian canals through which blood vessels can pass.
Endoderm - The inner layer of cells of a gastrula that becomes the lining of the gut.
Endoparasite - A parasite living inside its host.
Endoplasmic reticulum - A membrane system of folded sacs and tunnels in the cytoplasm of a cell.
Endospores - A protective envelope formed by bacteria and some other organisms to survive adverse conditions.
End point - When a neutralization reaction becomes neutral, neither acid nor base, during a titration.
Endocarp - The inner layer of the pericarp.
Endosperm - The food reserve in a seed derived from fertilized polar nuclei.
Endothermic - A reaction that releases energy; body temperature holds at a constant level.
Energy - The ability to do work.
Energy level - An area around the nucleus of an atom where electrons are found.
Enthalpy - Heat content in matter.
Enthalpy of formation - The change in enthalpy when one mole of a compound is produced from the free elements.
Entropy - Disorder in matter.
Environmental resistance - The combination of factors that tend to prevent exponential growth.
Enzymes - Proteins that act as catalysts in the chemical reactions of cells.
Epicotyl - The part of a plant embryo that will form the leaf of the new plant.
Epidermis - The outer layer of cells.
Epigynous - A flower in which the ovary is situated below the calyx.
Epipetalous - A flower whose stamens are attached to the corolla.
EPR paradox - The Einstein Podolsky Rosen paradox is a thought experiment originally formulated in 1935 by Albert Einstein, Boris Podolsky, and Nathan Rosen to demonstrate (as they thought) the logical impossibility of quantum mechanics.
Equilibrium - When there is no net change in a system.
Equivalent - The amount of an acid (or base) that will give one mole of hydrogen (or hydroxide) ions.
Erosion - The physical removal and transportation of materials on the Earth's surface by wind and water.
Erythrocytes - Red blood cells.
Escape velocity - The velocity needed to escape the orbital gravity of a planet.
Essay - A short, written communication about a certain topic.
Estivation - A period of inactivity during the dry summer.
Estuary - A tide-influenced ecological system where a river meets the sea and fresh water mixes with salt water.
Ethics - A system defining right and wrong behaviors.
Eubacteria - The kingdom of bacteria commonly referred to as germs.
Eukaryotes - Cells that contain a nucleus and organelles surrounded by a membrane.
Euphotic zone - Upper, well lighted areas of open ocean water supporting photosynthetic algae.
Euryecious species - A widely distributed species with a high tolerance for many environmental factors.
Eustachian tube - Connects the middle ear with the mouth cavity.
Eutherians - Placental mammals.
Eutrophic - Refers to an environment with many nutrients supporting a variety of life.
Eutrophication - The increase of nutrients in an environment.
Evaporation - Random molecules leaving the surface of a liquid.
Evolution - The process by which living things change over a period of time.
Excretion - The process of removing metabilic wastes from the body.
Exocarp - The outermost layer of the pericarp, ectocarp.
Exothermic - A reaction that releases energy.
Exoskeleton - An external skeleton providing support and protection.
Experimental error - A comparison of an experimental value to a known value.
Explosive - A chemical that produces a sudden release of pressure, gas and/or heat when subjected to sudden shock, pressure or high temperature.
External respiration - The exchange of gases between atmosphere and blood.
Extrapolate - Extending a graph, along the same slope, above or below measured data.