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.