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

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

He went so far as to throw an “activation Party” in their honor just weeks before his September arrest.
The Father Who Made His Kids Have Sex With a Dog John L. Smith December 15, 2014

Historical Examples

The activation thereafter had to be done in special equipment permitting of fine control of temperature.
America’s Munitions 1917-1918 Benedict Crowell

How this activation is being aided and accelerated by another source of dynamic energy: irradiation from the sun.
The Brain Alexander Blade

The scientist set the controls and turned on the activation switch.
The Golden Skull John Blaine

It is usually measured by inserting materials of known composition and measuring their activation.
The Atomic Fingerprint Bernard Keisch

But non-military items also arrived for activation and test.
The Machine That Saved The World William Fitzgerald Jenkins

There is no known explanation for this type of activation of the enzyme.
The Chemistry of Plant Life Roscoe Wilfred Thatcher

activation meant turning them on and giving them a sort of basic training in the tasks they were designed to do.
The Machine That Saved The World William Fitzgerald Jenkins

Then, as Tony switched the activation circuit, the vertical line formed a pattern that varied in width from top to bottom.
The Golden Skull John Blaine

verb (transitive)
to make active or capable of action
(physics) to make radioactive
(chem)

to increase the rate of (a reaction)
to treat (a substance, such as carbon or alumina) so as to increase powers of adsorption

(physiol) to prepare by arousal (the body or one of its organs (e.g. the brain)) for action
to purify (sewage) by aeration
(US, military) to create, mobilize, or organize (a unit)
n.

1906, noun of action from activate (v.).
v.

1620s; see active + -ate (2). Related: Activated; activating.