Muggles' Guide to Harry Potter/Magic/Foe Glass

Overview
A Foe-Glass is an object somewhat like a mirror which reveals approaching enemies. The closer said enemies are, the more focused their images are.

Extended Description
The first time we see a Foe Glass, it is mounted on the wall of Professor Moody's office. While there is constant motion within the glass when Harry first sees it, everything he sees is indistinct, indicating that there are no enemies nearby.

Once Moody has revealed himself, Harry notices that there are three images in the glass that are steadily becoming more distinct; shortly before the door is broken down, he recognizes the images as Professor Dumbledore, Professor McGonagall, and Professor Snape. The image of the three stays in the mirror while Moody is questioned.

We see a Foe Glass, quite possibly the same one, mounted on the wall of the Room of Requirement when Dumbledore's Army meets there in Harry Potter and the Order of the Phoenix.