So big, so strange, or so extreme that your mind cannot make a picture of it. You use it when a thing is beyond what a normal person can hold in their head: unimaginable wealth, unimaginable pain, unimaginable cruelty. Notice that it goes with both wonderful things and terrible things — the word itself does not say good or bad, only "too much for the imagination". In real speech it is a strong word people use to show feeling, not a careful measurement. When a reporter says the damage was unimaginable, they do not mean it is literally impossible to imagine; they mean it is shocking, more than anyone expected. There is a second, cooler use meaning "impossible to believe would happen": "Twenty years ago, a phone like this was unimaginable." Here it is talking about what people at the time could not have predicted.