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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

The manager responded by directing the first teller to call the police and activate the alarm.
New Usual Suspects in U.S. Bank Heists: Women Michael Daly May 13, 2012

Buy a printed hardcover or paperback book for, say, $25 and then you could activate it as a digital file or downloadable audio.
Buy One Book, Get One Free Peter Osnos May 18, 2009

I read a study not long ago that chewing actually does activate your brain.
Elizabeth Gilbert: How I Write Noah Charney October 1, 2013

Perry will home in on the darkest corners of the Tea Party mind and work relentlessly to activate the demons that lurk there.
Perry: King of the Know-Nothings Michael Tomasky August 18, 2011

We now know that both of the so-called printer bombs employed circuit boards from cellphones to activate the detonators.
Al Qaeda’s Doomsday Plan Clive Irving November 1, 2010

Historical Examples

It could be induced to go into action and to activate the other plasmoids.
Legacy James H Schmitz

The males are few in number and lack the power to activate the disks.
Storm Over Warlock Andre Norton

It seemed to activate some hidden response in her, for she glowed with tenderness.
Martians Never Die Lucius Daniel

Furiously, he tried to summon some tiny bit of energy to activate the distorter.
The Weakling Everett B. Cole

A neutron moderator slows down the neutrons and thus makes them more likely to activate the calcium in the bones.
The Atomic Fingerprint Bernard Keisch

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)
v.

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