As a Noun:
1. The situation in which something cannot be done, cannot happen, or cannot be believed. English = impossibility.
As a Verb or Adjective:
1. To be something that cannot be done, cannot happen, or cannot be believed. English = (be) impossible.
1. Used alone in response to your addressee's comments to mean that you do not believe what they say is true, or possible, or that it really surprises you. English = 'Impossible', 'No way!', 'Incredible!', and so on.