Wave

a mechanism of energy transfer through a medium. The origin of the wave is vibrating particles, which store and release energy while their mean position remains constant as it is only the wave that travels. Waves can be classified as being either LONGITUDINAL WAVES, e.g. sound, or TRANSVERSE WAVES, e.g., light, depending on the direction of their vibrations. There is a basic wave equation that related the wavelength (λ), frequency(f), and speed (c) of the wave as c = fλ. All forms of waves have the following properties: diffraction; interference; reflection and refraction. ELECTROMAGNETIC WAVES have all of these properties but differ from ordinary waves, such as water waves, in that they can travel through a vacuum, e.g. outer space.

Taken from Dictionary of Science

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A wave is a disturbance that propagates through space and time, usually with transference of energy. A mechanical wave is a wave that propagates or travels through a medium due to the restoring forces it produces upon deformation. There also exist waves capable of traveling through a vacuum, including electromagnetic radiation and probably gravitational radiation. Waves travel and transfer energy from one point to another, often with no permanent displacement of the particles of the medium (that is, with little or no associated mass transport); they consist instead of oscillations or vibrations around almost fixed locations.

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Radioactivity

the giving out of particles (known as α or β particles and γ rays) by unstable substances that are disintegrating.

Taken from Dictionary of Science

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Radioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter nuclide. For example: a carbon-14 atom (the "parent") emits radiation and transforms to a nitrogen-14 atom (the "daughter"). This is a stochastic process on the atomic level, in that it is impossible to predict when a given atom will decay, but given a large number of similar atoms the decay rate, on average, is predictable.

The SI unit of activity is the becquerel (Bq). One Bq is defined as one transformation (or decay) per second. Since any reasonably-sized sample of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts on the order of GBq (gigabecquerel, 1 x 10⁹ decays per second) or TBq (terabecquerel, 1 x 10¹² decays per second) are commonly used. Another unit of radioactivity is the curie, Ci, which was originally defined as the amount of radium emanation (radon-222) in equilibrium with of one gram of pure radium, isotope Ra-226. At present it is equal, by definition, to the activity of any radionuclide decaying with a disintegration rate of 3.7 × 10¹⁰ Bq. The use of Ci is presently discouraged by the SI.

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Absolute Zero

the temperature at which the particles that make up matter have no energy at all whether due to heat or motion. It is theoretically given the value of -273.15°Celsius (-459.67° Fahrenheit).

Taken from Dictionary of Science
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Absolute zero is the temperature at which entropy reaches its minimum value. As implied by the laws of thermodynamics, absolute zero cannot be reached by artificial or natural means because this would require a system to be fully removed from the rest of the universe. A system at theoretical absolute zero possesses quantum mechanical zero-point energy. While all molecular motion does not cease at absolute zero, the system does not have enough energy for transference to other systems. It is therefore correct to say that molecular energy is minimal at absolute zero.

By international agreement, absolute zero is defined as precisely 0 K on the Kelvin scale and as −273.15° on the Celsius scale. Absolute zero is also precisely equivalent to 0 R on the Rankine scale (same as Kelvin but measured in Fahrenheit intervals) and −459.67° on the Fahrenheit scale. Though it is not theoretically possible to cool any substance to 0 K, scientists have made great advancements in achieving temperatures close to absolute zero, where matter exhibits quantum effects such as superconductivity and superfluidity.

For the kinematics of molecules at absolute zero on a larger scale, which is easier to understand, see kinetic energy.

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Radiation

the giving out of energy from a source, which may be in the form of ELECTROMAGNETIC WAVES (radio, light, X-rays, infrared rays, etc.), particles or sound waves.

Taken from Dictionary of Science

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In physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body. Non-physicists often associate the word with ionizing radiation (e.g., as occurring in nuclear weapons, nuclear reactors, and radioactive substances), but it can also refer to electromagnetic radiation (i.e., radio waves, infrared light, visible light, ultraviolet light, and X-rays) which can also be ionizing radiation, to acoustic radiation, or to other more obscure processes. What makes it radiation is that the energy radiates (i.e., it travels outward in straight lines in all directions) from the source. This geometry naturally leads to a system of measurements and physical units that are equally applicable to all types of radiation. Some radiations can be hazardous.

Taken from Wikipedia