A one-in-a-million event is an extraordinary occurrence that is extremely rare and unlikely to happen. It can also be referred to as a MIRACLE. The term is often used to describe a situation where the odds of something happening are so low that it is almost impossible for it to occur. In this case, the crossword solution for the term 'one-in-a-million event' is 'MIRACLE', which is a fitting description for such an event.