Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound in metabolism and is essential to the flow of energy in living cells. ADP consists of three important structural components: a sugar backbone attached to adenine and two phosphate groups bonded to the 5 carbon atom of ribose.

Actinium (Ac) is a silvery radioactive metallic element. Actinium glows in the dark due to its intense radioactivity with a blue light. Actinium was discovered in 1899 by Andre-Louis Debierne, a French chemist, who separated it from pitchblende. Friedrich Otto Giesel independently discovered actinium in 1902.

Acrylonitrile butadiene styrene (ABS) is a common thermoplastic polymer. Its glass transition temperature is approximately 105 °C (221 °F). ABS is amorphous and therefore has no true melting point.

Acquired immunodeficiency syndrome (AIDS) is a chronic, potentially life-threatening condition caused by the human immunodeficiency virus (HIV). By damaging your immune system, HIV interferes with your body's ability to fight infection and disease.

AACC (formerly known as the American Association for Clinical Chemistry) is a global scientific society dedicated to clinical laboratory science and its application to healthcare. AACC's current president is Carmen Wiley, PhD, and the association headquarters are located in Washington, D.C. (United States).

Aldehyde oxidase (AO) is a metabolizing enzyme, located in the cytosolic compartment of tissues in many organisms. AO catalyzes the oxidation of aldehydes into carboxylic acid, and in addition, catalyzes the hydroxylation of some heterocycles. It can also catalyze the oxidation of both cytochrome P450 (CYP450) and monoamine oxidase (MAO) intermediate products. AO plays an important role in the metabolism of several drugs.

Aldehyde dehydrogenase (ALDH) are a group of enzymes that catalyse the oxidation of aldehydes. They convert aldehydes to carboxylic acids. The oxygen comes from a water molecule. To date, nineteen ALDH genes have been identified within the human genome.

Alcohol by Weight (ABW) is a measure of how much alcohol is in a beverage, expressed as a percentage of the total beverage weight. The US often quotes alcohol values by weight (not volume) and the Bureau of Alcohol, Tobacco and Firearms (BATF) uses the weight measurement.

Alcohol by volume (abbreviated as ABV, abv, or alc/vol) is a standard measure of how much alcohol (ethanol) is contained in a given volume of an alcoholic beverage (expressed as a volume percent).

Adenosine monophosphate (AMP) is a nucleoside phosphate in being comprised of a ribonucleoside and one phosphate group. It means, it has a ribose as its sugar and one phosphate group attached. Its nucleoside contains a purine base, i.e. an adenine attached to the ribose sugar. It has only one phosphate group attached to the nucleoside.

Astatine is a chemical element with the symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-lived; the most stable is astatine-210, with a half-life of 8.1 hours.

Aspirin, also known as acetylsalicylic acid (ASA), is a medication used to reduce pain, fever, or inflammation. Specific inflammatory conditions which aspirin is used to treat include Kawasaki disease, pericarditis, and rheumatic fever.

Aspartate (ASP) is a vitamin-like substance called an amino acid. As a dietary supplement, aspartate is combined with minerals and is available as copper aspartate, iron aspartate, magnesium aspartate, manganese aspartate, potassium aspartate, and zinc aspartate.

Aromatic polyimides (API) are a special group of engineering organic materials, which are classified as high performance polymers, because of their excellent thermal, mechanical and chemical properties.

Antidiuretic hormone (ADH) is a hormone that helps your kidneys manage the amount of water in your body. The ADH test measures how much ADH is in your blood. This test is often combined with other tests to find out what is causing too much or too little of this hormone to be present in the blood.

Analytical Reagent (AR) is a chemical compound of a known high standard of purity.

An atomic mass unit (AMU) is defined as a mass equal to one twelfth the mass of an atom of carbon-12. The mass of any isotope of any element is expressed in relation to the carbon-12 standard. For example, one atom of helium-4 has a mass of 4.0026 amu. An atom of sulfur-32 has a mass of 31.972 amu.

An airborne early warning and control (AEW&C) system is an airborne radar picket system designed to detect aircraft, ships, vehicles, missiles, and other incoming projectiles at long ranges and perform command and control of the battlespace in an air engagement by directing fighter and attack aircraft strikes. AWACS (Airborne Warning and Control System) is the name of the specific system installed in the E-3 and Japanese Boeing E-767 AEW&C airframes, but is often used as a general synonym for AEW&C.

Ammonium sulfate is an inorganic sulfate salt obtained by reaction of sulfuric acid with two equivalents of ammonia. A high-melting (decomposes above 280°C) white solid which is very soluble in water (70.6 g/100 g water at 0°C; 103.8 g/100 g water at 100°C), it is widely used as a fertilizer for alkaline soils.

Ammonium nitrate (AN) is a crystal-like white solid which is made in large industrial quantities. Its biggest use is as a source of nitrogen for fertiliser, but it is also used to create explosives for mining.