to make active; cause to function or act.

Physics .

to render more reactive; excite

to activate a molecule.

to induce radioactivity.

to aerate (sewage) in order to accelerate decomposition of impure organic matter by microorganisms.

Chemistry .

to make (carbon, a catalyst, molecules, etc.) more active.

to hasten (reactions) by various means, as heating.

to place (a military unit or station) on an active status in an assigned capacity.