actinium(A radioactive element of the actinide series, occurring as a decay product of uranium. It is used as an alpha particle source and in neutron production)
activated carbon(A powdered, granular or pelleted form of amorphous carbon characterized by a very large surface area per unit volume because of an enormous number of fine pores)
activated sludge(Sludge that has been aerated and subjected to bacterial action; used to speed breakdown of organism matter in raw sewage during secondary waste treatment)
active participation(The involvement, either by an individual or a group of individuals, in their own governance or other activities, with the purpose of exerting influence)
additional packaging(Additional packaging around the normal sales packaging. For example as protection against theft or for the purpose of advertising; the customer may leave the additional packaging in the shop for waste collection)