The direct opposite of a black hole. Whereas a black hole always sucks up matter, white holes always spew out matter. Just like wormholes, they're mathematically possible, but they don't exist in the real universe. At least, not in the dimensions we can perceive... Bwuhahahaha! *Twilight Zone music kicks in*
White Holes are the exact opposite of black holes.
The counterpart to a black hole. In a black hole matter collapses inward to a singularity; in a white hole, matter pours out from a singularity. There are many similarities between the Big Bang and a white hole.
As well as black holes and white holes, relativists sometimes talk about grey holes.