1. To set in motion: Some students at school played a prank and activated the fire alarm.
2. To organize or to create a military unit: The governor had to activate the National Guard to help the people after the hurricane.
3. To treat (sewage) with aeration and bacteria to aid decomposition: Air and bacteria helped to activate and facilitate the decaying process of the compost in the garden so that it could be used later as very good soil.
4. In chemistry, to accelerate a reaction in something: In class the teacher showed the students how heat helped to activate certain substances.
5. In physics, to make a substance radioactive: Jane learned in her physics class that it was possible to make or to activate something radioactive by exposing it to the influence of a radioactive matter.
6. In biology, to convert certain biological compounds into a biologically functioning derivative: The students read about how enzymes were activated, or how a molecule or cell was activated into a structure that performed a certain action.

A highly porous form of carbon, typically from wood, lignite, coal, or coconut shells: Activated carbon is widely used as a filtration medium with the purpose of removing taste and odor from water by absorbing organic compounds.

The semi-liquid, microbe-rich sediment that is added to secondary stage sewage material: Activated sludge is the waste matter that has been aired and churned in order to facilitate the growth of useful microorganisms which then decompose organic substances.

A widely used method for sewage treatment that raises the level of biological activity: An activated sludge process takes place due to an increase of contact between wastewater and actively growing micro-organisms.

1. A reserve battery with an aqueous electrolyte kept in a separate chamber, from which a mechanism forces it into the cells of the battery for activation.
2. A reserve battery in which an aqueous electrolyte is stored in a separate chamber, and a mechanism, which may be operated from a remote location, drives the electrolyte out of the reservoir and into the cells of the battery for activation.

1. A combination of thermal cells.
2. A voltage source consisting of a number of bimetallic junctions connected to produce a voltage when heated by a flame.
3. A high-temperature, molten-salt primary battery in which the electrolyte is a solid, non-conducting inorganic salt at ambient temperatures.d

When power is required, an internal pyrotechnic heat source is ignited to melt the solid electrolyte which allows electricity to be generated electrochemically for periods from a few seconds to an hour.

This process is often used for military applications; such as, missiles, torpedoes, and space missions.

A diverse group of devices employed to transform heat for a useful purpose; such as, energy recovery, heating, cooling, humidity control, or thermal storage.