A collector was an engineering tool that could gather and store energy.
In 1986, it was used by Nyota Uhura and Pavel Chekov for collecting high energy photons from a nuclear fission reactor on board the USS Enterprise. Later, the photons from this device were streamed back into the dilithium chamber of the HMS Bounty. This was done to re-crystallize the dilithium crystals. (Star Trek IV: The Voyage Home)