Vitamins are complex organic compounds that occur naturally in plants and animals. People and other animals need these compounds in order to maintain life functions and prevent diseases.


Vivisection (pronounced vih-vih-SEK-shun) literally means the dissection or cutting of a living animal. The term has come to apply to any and all types of experiments on live animals, and it is a term to which many scientists object.


A volcano is a hole in Earth's surface through which magma (called lava when it reaches Earth's surface), hot gases, ash, and rock fragments escape from deep inside the planet. The word volcano also is used to describe the cone of erupted material (lava and ash) that builds up around the opening.


Volume is the amount of space occupied by an object or a material. Volume is said to be a derived unit, since the volume of an object can be known from other measurements.

Waste Management

Waste management is the handling of discarded materials. The term most commonly applies to the disposition of solid wastes, which is often described as solid waste management.


Water is an odorless, tasteless, transparent liquid that appears colorless but is actually very pale blue. The color is obvious in large quantities of water such as lakes and oceans.

Wave Motion

Wave motion is a disturbance that moves from place to place in some medium, carrying energy with it. Probably the most familiar example of wave motion is the action of water waves.


Weather is the state of the atmosphere at any given time and place, determined by such factors as temperature, precipitation, cloud cover, humidity, air pressure, and wind. The study of weather is known as meteorology.

Weather Forecasting

Weather forecasting is the attempt by meteorologists to predict weather conditions that may be expected at some future time. Weather forecasting is the single most important practical reason for the existence of meteorology, the study of weather, as a science.


Wetlands are low-lying ecosystems that are saturated with water at or close to the surface. (An ecosystem consists of all the animals, plants, and microorganisms that make up a particular community living in a certain environment.) The most common types of wetlands are swamps, marshes, and bogs.

White Dwarf

Black dwarf: Cooling remnants of a white dwarf that has ceased to glow.


Wind refers to any flow of air above Earth's surface in a roughly horizontal direction. A wind is always named according to the direction from which it blows.

X Ray

X rays are a form of electromagnetic radiation with wavelengths that range from about 10−7 to about 10−15 meter. No sharp boundary exists between X rays and ultraviolet radiation on the longer wavelength side of this range.

X-ray Astronomy

Stars and other celestial objects radiate energy in many wavelengths other than visible light, which is only one small part of the electromagnetic spectrum. At the low end (with wavelengths longer than visible light) are low-energy infrared radiation and radio waves.


Yeast are microscopic, single-celled organisms that are classified in the family Fungi. Individual yeast cells multiply rapidly by the process of budding, in which a new cell begins as a small bulge along the cell wall of a parent cell.


The most common meaning of the term zero is the absence of any magnitude or quantity. For example, a person might say that he or she has zero children, meaning that he or she has no children.