## Correlation

As used in mathematics, correlation is a measure of how closely two variables change in relationship to each other. For example, consider the variables height and age for boys and girls.

## Cosmic Ray

Cosmic rays are invisible, highly energetic particles of matter reaching Earth from all directions in space. Physicists divide cosmic rays into two categories: primary and secondary.

## Cosmology

Cosmology is the study of the origin, evolution, and structure of the universe. This science grew out of mythology, religion, and simple observations and is now grounded in mathematical theories, technological advances, and space exploration.

## Cotton

Cotton is a fiber obtained from various species of woody plants and is the most important and widely used natural fiber in the world. The leading cotton-producing countries are China (the world's biggest producer), the United States, India, Pakistan, Brazil, and Egypt.

## Coulomb

A coulomb (abbreviation: C) is the standard unit of charge in the metric system. It was named after French physicist Charles A.

## Crops

Crops are plants or animals or their products cultivated (grown, tended, and harvested) by humans as a source of food, materials, or energy. Humans are rather particular in their choice of crops.

## Crustaceans

The crustaceans are a group of animals that belong to the class Crustacea in the phylum Arthropoda (organisms with segmented bodies, jointed legs or wings, and an external skeleton). The class includes a wide variety of familiar animals, such as barnacles, crabs, crayfish, copepods, shrimp, prawns, lobsters, water fleas, and wood lice.

## Cryobiology

Cryobiology is the study of the effects of very low temperatures on living things. Cryobiology can be used to preserve, store, or destroy living cells.

## Cryogenics

Cryogenics is the science of producing and studying low-temperature conditions. The word cryogenics comes from the Greek word cryos, meaning "cold," combined with a shortened form of the English verb "to generate." It has come to mean the generation of temperatures well below those of normal human experience.

## Crystal

A crystal is a solid whose particles are arranged in an orderly, repeating, geometric pattern. Crystals come in all sizes and shapes.

## Currents, Ocean

Currents are steady, smooth movements of water following either a straight or circular path. All of the water in the oceans on Earth are in constant circulation.

## Cybernetics

Cybernetics is the study of communication and control processes in living organisms and machines. Cybernetics analyzes the ability of humans, animals, and some machines to respond to or make adjustments based upon input from the environment.

## Cyclamate

Cyclamate is the name given to a family of organic compounds that became popular in the 1950s as artificial sweeteners. They are about 30 times as sweet as ordinary table sugar (sucrose) but have none of sugar's calories.

## Cyclone and Anticyclone

A cyclone is a storm or system of winds that rotates around a center of low atmospheric pressure. An anticyclone is a system of winds that rotates around a center of high atmospheric pressure.

## Dam

Dams are structures that hold back water in a stream or river, forming a lake or reservoir behind the wall. Dams are used as flood control devices and as sources of hydroelectric power and water for crops.

## Dark Matter

Dark matter is the term astronomers use to describe material in the universe that does not emit or reflect light and is, therefore, invisible. Stars, nebulae, and galaxies are examples of luminous objects in the sky.

## Dating Techniques

Dating techniques are procedures used by scientists to determine the age of an object or a series of events. The two main types of dating methods are relative and absolute.

## DDT (dichlorodiphenyltrichloroethane)

DDT is a synthetic chemical compound once used widely in the United States and throughout the world as a pesticide (a chemical substance used to kill weeds, insects, rodents, or other pests). It is probably best known for its dual nature: although remarkably effective in destroying certain living things that are harmful to plants and animals, it can also be extremely dangerous to humans and the environment.

## Dementia

Dementia is a decline in a person's ability to think and learn. It is an irreversible mental condition.

## Density

Density is defined as the mass of a unit volume of some material. The term unit volume means the amount contained in one volumetric unit of measurement: one cubic foot, one liter, or one milliliter, for example.

## Dentistry

Dentistry is the medical field concerned with the treatment and care of the teeth, the gums, and the oral cavity. This includes treating teeth damaged by tooth decay, accidents, or disease.

## Depression

Depression is one of the most common mood disorders. Everyone experiences depressed moods from time to time.

## Desert

A desert is an arid land area that generally receives less than 10 inches (250 millimeters) of rainfall per year. What little water it does receive is quickly lost through evaporation.

## Diabetes Mellitus

Diabetes mellitus is a disease caused by the body's inability to use the hormone insulin. Insulin is normally produced in the pancreas, a gland attached to the small intestine.

## Diagnosis

Diagnosis is the process of identifying a disease or disorder in a person by examining the person and studying the results of medical tests.

## Dialysis

Dialysis is a process by which small molecules in a solution are separated from large molecules. Dialysis has a number of important commercial and industrial applications and plays a crucial role in maintaining the health of humans.

## Diesel Engine

A diesel engine is a type of internal-combustion engine developed by German engineer Rudolf Diesel (1858–1913) in the late nineteenth century. His original design called for the use of coal dust as fuel, but most modern diesel engines burn low-cost fuel oil.

## Diffraction

Diffraction is the bending of waves (such as light waves or sound waves) as they pass around an obstacle or through an opening. Anyone who has watched ocean waves entering a bay or harbor has probably witnessed diffraction.