Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface.

A feature allowing the user to adjust the setpoint of a variable that will invoke an action on another variable.

In thermodynamics, an adiabatic process is a type of thermodynamic process which occurs without transferring heat or mass between the system and its surroundings.

Acute radiation syndrome (ARS), also known as radiation sickness or radiation poisoning, is a collection of health effects that are caused by being exposed to high amounts of ionizing radiation, in a short period of time.

An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve.

An active stall system reads the power output the way a pitch-controlled system does, but instead of pitching the blades out of alignment with the wind, it pitches them to produce stall.

Solar hot water systems use pumps or fans to circulate fluid (often a mixture of water and glycol to prevent freezing during winter periods) or air, through solar collectors, and are therefore classified under active solar technology.

The length of the U-bend in the borehole below the header trench (usually 4 to 6 feet less than the total borehole length from the surface).

Activation products are materials made radioactive by neutron activation.

In chemistry and physics, activation energy is the minimum amount of energy that must be provided to compounds to result in a chemical reaction.

Actinometers are instruments used to measure the heating power of radiation.

In a fast neutron reactor, the minor actinides produced by neutron capture of uranium and plutonium can be used as fuel.

The actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.

Acid rain is a form of precipitation which has high sulfuric and nitric acids which can occur in the form of a fog or snow caused by fossils that are burned by humans for energy.

An accumulator is an energy storage device: a device which accepts energy, stores energy, and releases energy as needed.

Accreditation is a third-party attestation related to a conformity assessment body (such as certification body, inspection body or laboratory) conveying formal demonstration of its competence to carry out specific conformity assessment tasks (such as certification, inspection and testing).

Acceptance angle is the maximum angle at which incoming sunlight can be captured by a solar concentrator.

An accelerator-driven subcritical reactor is a nuclear reactor design formed by coupling a substantially subcritical nuclear reactor core with a high-energy proton or electron accelerator.

Acceleration is the rate of change of the velocity of an object with respect to time.

Accelerated life testing is the process of testing a product by subjecting it to conditions (stress, strain, temperatures, voltage, vibration rate, pressure etc.) in excess of its normal service parameters in an effort to uncover faults and potential modes of failure in a short amount of time.