A Brocken spectre is an apparently gigantic shadow cast in fog or mist, often surrounded by an aura. According to Wikipedia:
The "spectre" appears when the sun shines from behind a climber who is looking down from a ridge or peak into mist. The light projects the climber's shadow forward through the mist, often in an odd triangular shape due to perspective. The apparent magnification of size of the shadow is an optical illusion that occurs when the observer judges his shadow on relatively nearby clouds to be at the same distance as faraway land objects seen through gaps in the clouds, or when there are no reference points at all by which to judge its size. The shadow also falls on water droplets of varying distances from the eye, confusing depth perception . The ghost can appear to move (sometimes quite suddenly) because of the movement of the cloud layer.
The Nonist has an article on Brocken spectres today, with a good collection of photos.