The Deluminator or, as it is known in Harry Potter and the Philosopher's Stone, the Put-Outer, is a device which Albus Dumbledore invented. It can turn off Muggle lights.
It is first discovered (as the "Put-Outer") in "Something Peculiar is Happening".
From the Story
|“||Albus Dumbledore uses this item – which looks like a silver cigarette lighter – to turn out all the lights on Privet Drive. Each time he clicks it, a light flickers out, eventually leaving the street in near-total darkness.||”|