It used to be that in Einstein's theory of general relativity, that solutions to his equation for gravity and space-time only allowed two kinds of singularities. The familiar 'black hole' where your journey would be one-way in, with no possibility of escape; and the 'white hole' in which you could never enter it but only leave, which means your journey had to start inside it! Then, in 1975, physicist Stephen Hawking announced that black holes could evaporate because of the way that the gravitational field near a black hole's event horizon affects the vacuum of space in a quantum mechanical way. Well, if a black hole could emit particles then it wasn't really completely black, so some people began to call this a 'grey hole'. Today, no one really uses this term because 'white' and 'black' have to do with a fundamental property of the solutions of Einstein's equations, and Hawking's solution is just a minor 'tweak'.