Absolute Humidity - Is the amount of humidity in the air, indicated by grains

Absolute Pressure - is the sum of gauge and atmospheric pressure (psia). 

Absolute Temperature - Is temperature measured on the Kelvin scale

Absolute Zero - The lowest temperature theoretically attainable on the Kelvin scale . Approximately  -273.16^o C

^{Absolute humidity - The humidity of the air measured buy the number of grains of water vapor present in one cubic meter of air.} 

Absorbent - A substance with the ability to absorb another substance.

Absorption-  in physics, the taking up of light, heat, or other  energy by molecules. The absorbed energy is converted into heat;  Absorption in chemistry is the taking up of one substance by another. For example, a gas such as oxygen may be absorbed, or dissolved, in water. In the HVAC industry we absorbed heat energy from the medium being cooled and transfer that energy in the refrigerant.

Active Cooling - Compressor driven air-conditioning.

Accumulator - A shell device installed in the suction line of a HVAC system to prevent liquids from entering the compressor.

Acrolein - An Agent added to methyl chloride, to make you aware of refrigerant leaks.

Activated Alumina - a type of aluminum oxide that absorbs moisture. (used in refrigerant driers).

Activated Carbon - A processed carbon used in filter driers, commonly used in air filters to clean the air.

Adiabatic - A change in gas condition where no heat is added or removed except in the form of work.

Adsorption-Adsorption, which is often confused with absorption, refers to the adhering of molecules of gases and liquids to the surfaces of porous solids. Adsorption is a surface phenomenon; absorption is an intermingling or interpenetration of two substances.

Air Balance- To distribute air through a system to precisely match the required amount.

Air Conditioner - A device used to control temperature and humidity of the air.

Air Conditioning - To condition air,   transfer of heat and humidity from one medium to an other . 

Air Handler - A fan-blower , heat transfer coil and housing parts of a system.

Air, Standard - Air having a temperature of 68^oF (20^oC), a relative humidity of 36% at 14.7 psia.

Air Infiltration - The unwanted entrance of air due to suck things as leakage, temperature difference or wind.

Air vent - A valve , either manual or automatic that is used to remove unwanted air from the highest point of a piping system.

Air to Air -  both the condensing and evaporating mediums are air.

Alcohol Brine - Water and alcohol solution witch remains a liquid below 32^oF.

Alternating Current - electrical current that reverses direction periodically. 

Ambient temperature - The temperature surrounding an object.

Ammeter - An Instrument for measuring the current in amps, milliamps or micro amps.

Ampere - the current required to produce a certain amount of force between two wires. 

Amplitude - The highest value reaches by voltage,  current or power during a complete cycle.

Analogue - A system in which data is represented as a continuously varying voltage.

Anemometer- an instrument that measures air speed.

Anticipation - to realize in advance, ie- a heat anticipator on a thermostat is used to shut down a furnace prior to the set point, as we are anticipating that the furnace will continue to run until the heat exchange is cooled and we will be at set point buy that time .

Anode - Positive terminal of electrolytic cell.

ASHRAE - American Society of Heating Refrigeration and Air Conditioning Engineers.

Aspect Ratio - The ratio of length to width of a rectangular air grill or duct.

Aspiration - The movement is a fluid produced by suction.

Atmospheric Pressure - Is the pressure that the atmosphere exerts on us. At sea level the is 14.7 psig also written as 29.92 " Hg.

Atmospheric Condenser - A condenser operated with water that is exposed to the atmosphere.

Atmospheric Dust Spot Efficiency - the measurement of a devices ability to remove dust from the air.

Automatic Expansion Valve - A pressure actuated metering device to regulate the flow of refrigerant into the evaporator according to the evaporator pressure.

Azeotropes -  refrigerants that are mixtures or blends of different refrigerants that are at the same ratio in vapor as well as in liquid state.

Azeotropes - (Near) - A zeotrope having a temperature glide sufficiently small that it is inconsequential.

Azeotropic mixture - is a combination of different refrigerants to mask one with desirable refrigerant properties, an example of this is R-502 witch is a mixture of 48.8% R-22 and 51.2% R-115.