An allergy is an extreme or overly sensitive response of the immune system to harmless substances in the environment. The immune system launches a complex series of actions against an irritating substance, referred to as an allergen.


An alloy is a mixture of two or more metals. Some familiar examples of alloys include brass, bronze, pewter, cast and wrought iron, steel, coin metals, and solder (pronounced SOD-der; a substance used to join other metallic surfaces together).

Alternative Energy Sources

Alternative energy is energy provided from sources other than the three fossil fuels: coal, oil, and natural gas. Alternative sources of energy include nuclear power, solar power, wind power, water power, and geothermal energy, among others.

Alternative Medicine

Alternative medicine is the practice of techniques to treat and prevent disease that are not generally accepted by conservative modern Western medicine. These techniques include homeopathy, acupuncture, herbal medicine, yoga, meditation, chiropractic, massage therapy, biofeedback, naturopathy, and many others.

Aluminum Family

The aluminum family consists of elements in Group 13 of the periodic table: boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl). The family is usually named after the second element, aluminum, rather than the first, boron, because boron is less typical of the family members than is aluminum.

Alzheimer's Disease

Alzheimer's is a brain disease in which damaged and dying brain cells cause devastating mental deterioration over a period of time. Often confused with senility (mental and physical deterioration associated with old age), its symptoms include increasingly poor memory, personality changes, and loss of concentration and judgment.

Amino Acid

Amino acids are simple organic compounds made of carbon, hydrogen, oxygen, nitrogen, and, in a few cases, sulfur. Amino acids bond together to form protein molecules, the basic building blocks of all living things.


An amoeba (pronounced uh-MEE-buh) is any of several tiny, one-celled protozoa in the phylum (or primary division of the animal kingdom) Sarcodina. Amoebas live in freshwater and salt water, in soil, and as parasites in moist body parts of animals.


Amphibians are cold-blooded animals that possess backbones and display features that lie between those of fish and reptiles. They spend time both in water and on land.


Anatomy is a branch of biology that deals with the structure of plants and animals. Comparative anatomy is a related field in which the structures of different animals are studied and compared.


Anesthesia is the term given to the loss of feeling or sensation. In medical terms, it is the method of decreasing sensitivity to pain in a patient so that a medical procedure may be performed.


Animals are creatures in the kingdom Animalia, one of the five major divisions of organisms. They are multicelled, eukaryotic (pronounced yookar-ee-AH-tik) organisms, meaning their cells contain nuclei and other structures called organelles, all of which are enclosed by thin membranes.


Antarctica, lying at the southernmost tip of the world, is the coldest, driest, and windiest continent. Ice covers 98 percent of the land, and its 5,100,000 square miles (13,209,000 square kilometers) cover nearly one-tenth of Earth's land surface, about the same size as Europe and the United States combined.


An antenna is a device used to transmit and receive electromagnetic waves, such as radio waves and microwaves. Antennas are found in a great variety of communication devices, including radios and television sets, weather radar systems, satellite communications systems, and radio astronomy research centers.


The discovery of antibiotics greatly improved the quality of human life in the twentieth century. Antibiotics are drugs such as penicillin (pronounced pen-ih-SILL-in) and streptomycin (pronounced strep-toe-MY-sin) used to fight infections and infectious diseases caused by bacteria.

Antibody and Antigen

Antibodies, also called immunoglobulins, are proteins manufactured by the body that help fight against foreign substances called antigens. When an antigen enters the body, it stimulates the immune system to produce antibodies.


Antiparticles are subatomic particles that are the opposite of other subatomic particles in some way or another. In the case of the antielectron and the antiproton, this difference is a matter of charge.


Antisepsis (from the Greek, anti, meaning "against," and sepsis, meaning "decay") is the destruction or control of the growth of microorganisms on living tissue. Antiseptics are the substances that carry out antisepsis.


Aquaculture refers to the breeding of fish and other aquatic animals for use as food. Aquaculture has long been practiced in China and other places in eastern Asia, where freshwater fish have been grown as food in managed ponds for thousands of years.


Arachnids (pronounced uh-RACK-nidz; class Arachnida) form the second-largest group of land arthropods (phylum Arthropoda) after the class Insecta. There are over 70,000 species of arachnids that include such familiar creatures as scorpions, spiders, harvestmen or daddy longlegs, and ticks and mites, as well as the less common whip scorpions, pseudoscorpions, and sun spiders.


Archaeoastronomy is the study of the astronomy of ancient people. As a field it is relatively young, having formally begun only in the 1960s.


Archaeology is the scientific recovery and study of artifacts (objects made by humans) of past cultures. By examining artifacts and how and where they were found, archaeologists try to reconstruct the daily lives of the people of those cultures.


Arithmetic is a branch of mathematics concerned with the addition, subtraction, multiplication, division, and extraction of roots of certain numbers known as real numbers. Real numbers are numbers with which you are familiar in everyday life: whole numbers, fractions, decimals, and roots, for example.


Arthritis is a general term meaning an inflammation of a bone joint. More than 100 diseases have symptoms of bone joint inflammation or injury.


Arthropods are invertebrate (without a backbone) animals of the phylum Arthropoda that have a segmented body, jointed legs, and a tough outer covering or exoskeleton. They include insects, crustaceans (lobsters, crabs, shrimp, crayfish), millipedes, centipedes, horseshoe crabs, arachnids (spiders, ticks, and mites) and sea spiders.

Artificial Fibers

An artificial fiber is a threadlike material invented by human researchers. Such fibers do not exist naturally.

Artificial Intelligence

Artificial intelligence (AI) is a subfield of computer science that focuses on creating computer software that imitates human learning and reasoning. Computers can out-perform people when it comes to storing information, solving numerical problems, and doing repetitive tasks.


Asbestos is the general name for a wide variety of silicate minerals, primarily silicates of calcium, magnesium, and iron. Silicates are commonly occurring minerals that all contain a characteristic combination of silicon and oxygen similar to that found in silicon dioxide (sand).