1. [adj]  (military) set up and placed on active assignment; "a newly activated unit"
2. [adj]  rendered active; e.g. rendered radioactive or luminescent or photosensitive or conductive
3. [adj]  (physics) of e.g. a molecule; made reactive or more reactive
4. [adj]  (of sewage) treated with aeration and bacteria to aid decomposition

ACTIVATED is a 9 letter word that starts with A.