Roger Woodham replies:'Incredible' and 'unbelievable'

These two adjectives, incredible and unbelievable, are quite interchangeable and to these two you could add a third: unimaginable.

They all describe things or events which are so amazing that they cannot be imagined or believed.

Adverbial forms are incredibly, unbelievably and unimaginably.

 

You can substitute any of these adjectives or adverbs under discussion as you wish: