A mutation is a permanent change in a gene that is passed from one generation to the next. An organism born with a mutation can look very different from its parents.

Natural Gas

Natural gas is a fossil fuel. Most scientists believe natural gas was created by the same forces that formed oil, another fossil fuel.

Natural Numbers

The natural numbers are the ordinary numbers, 1, 2, 3, etc., with which we count. They are sometimes called the counting numbers.

Nautical Archaeology

Nautical archaeology (pronounced NAW-tih-kul ar-kee-OL-low-jee) is the science of finding, collecting, preserving, and studying human objects that have become lost or buried under water. It is a fairly modern field of study since it depends primarily on having the technology both to locate submerged objects and to be able to remain underwater for some time to do real work.


Bright or dark clouds hovering in the interstellar medium (the space between the stars) are called nebulae. Nebula, Latin for "cloud," is a visual classification rather than a scientific one.


Neptune, the eighth planet away from the Sun, was discovered in 1846 by German astronomer Johann Galle, who based his finding on the mathematical predictions of French astronomer Urbain Le Verrier and English astronomer John Couch Adams. Because Neptune is so far way from the Sun—about 2.8 billion miles (4.5 billion kilometers)—it is difficult to observe.

Nervous System

The nervous system is a collection of cells, tissues, and organs through which an organism receives information from its surroundings and then directs the organism as to how to respond to that information. As an example, imagine that a child accidentally touches a very hot piece of metal.


A neutron is one of two particles found inside the nucleus (central part) of an atom. The other particle is called a proton.

Neutron Star

A neutron star is the dead remnant of a massive star. A star reaches the end of its life when it uses up all of its nuclear fuel.

Nitrogen Cycle

The term nitrogen cycle refers to a series of reactions in which the element nitrogen and its compounds pass continuously through Earth's atmosphere, lithosphere (crust), and hydrosphere (water component). The major components of the nitrogen cycle are shown in the accompanying figure.

Nitrogen Family

The nitrogen family consists of the five elements that make up Group 15 of the periodic table: nitrogen, phosphorus, arsenic, antimony, and bismuth. These five elements share one important structural property: they all have five electrons in the outermost energy level of their atoms.

Noble Gases

The noble gases are the six elements that make up Group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). At one time, this family of elements was also known as the rare gases.

North America

North America, the world's third-largest continent, encompasses an area of about 9,400,000 square miles (24,346,000 square kilometers). This landmass is occupied by the present-day countries of Canada, the United States, Mexico, Guatemala, Belize, El Salvador, Honduras, Nicaragua, Costa Rica, and Panama.


The word nova, Latin for "new," was assigned by ancient astronomers to any bright star that suddenly appeared in the sky.

Nuclear Fission

Nuclear fission is a process in which the nucleus of a heavy atom is broken apart into two or more smaller nuclei. The reaction was first discovered in the late 1930s when a target of uranium metal was bombarded with neutrons.

Nuclear Fusion

Nuclear fusion is the process by which two light atomic nuclei combine to form one heavier atomic nucleus. As an example, a proton and a neutron can be made to combine with each other to form a single particle called a deuteron.

Nuclear Medicine

Nuclear medicine is a special field of medicine in which radioactive materials are used to conduct medical research and to diagnose (detect) and treat medical disorders. The radioactive materials used are generally called radionuclides, meaning a form of an element that is radioactive.

Nuclear Power

Nuclear power is any method of doing work that makes use of nuclear fission or fusion reactions. In its broadest sense, the term refers both to the uncontrolled release of energy, as in fission or fusion weapons, and to the controlled release of energy, as in a nuclear power plant.

Nuclear Weapons

Nuclear weapons are destructive devices that derive their power from nuclear reactions. The term weapon refers to devices such as bombs and warheads designed to deliver explosive power against an enemy.

Nucleic Acid

A nucleic acid is a complex organic compound found in all living organisms. Nucleic acids were discovered in 1869 by the Swiss biochemist Johann Friedrich Miescher (1844–1895).

Number Theory

Number theory is the study of natural numbers. Natural numbers are the counting numbers that we use in everyday life: 1, 2, 3, 4, 5, and so on.

Numeration Systems

Numeration systems are methods for representing quantities. As a simple example, suppose you have a basket of oranges.


The term nutrition refers to the sum total of all the processes by which an organism takes in and makes use of the foods it needs to survive, grow, move, and develop. The word nutrition is also used to refer to the study of the substances an organism needs in order to survive.


An obsession is a persistent (continuous) and recurring thought that a person is unable to control. A person suffering from obsessive thoughts often has symptoms of anxiety (uneasiness or dread) or emotional distress.


Oceans are large bodies of salt water that surround Earth's continents and occupy the basins between them. The four major oceans of the world are the Atlantic, Arctic, Indian, and Pacific.


Oceanography is the scientific study of the oceans, which cover more than 71 percent of Earth's surface. It is divided into four major areas of research: physical, chemical, biological, and geological.

Ocean Zones

Ocean zones are layers within the oceans that contain distinctive plant and animal life. They are sometimes referred to as ocean layers or environmental zones.

Oil Drilling

Oil or petroleum (also known as crude oil) is a fossil fuel found largely in vast underground deposits. Oil and its byproducts (natural gas, gasoline, kerosene, asphalt, and fuel oil, among others) did not have any real economic value until the middle of the nineteenth century when drilling was first used as a method to obtain it.