White Hole: the opposite of a
black hole.
Since a
black hole is a region of
space from which nothing can
escape, the
time-reversed version of a black hole
is a region of space into which nothing can fall. In fact, just as a black hole can only suck things in, a white hole can only
spit
things out.
White holes are a perfectly valid
mathematical solution to the equations of
general relativity, but that doesn't mean that they actually exist in nature. In fact, they almost certainly do not exist, since there's no way to produce one. (Producing a white hole is just as impossible as destroying a black hole, since the two processes are time-reversals of each other, or so they theorize)