Dictionary:

astonishment

  (ə-stŏn'ĭsh-mənt)

1. Great surprise or amazement.
2. A cause of amazement; a marvel.

noun

1. The emotion aroused by something awe-inspiring or astounding: amaze, amazement, awe, marvel, wonder, wonderment. Archaic admiration, dread. See excite/bore/interest, feelings.
2. One that evokes great surprise and admiration: marvel, miracle, phenomenon, prodigy, sensation, stunner, wonder, wonderment. Idioms: one for the books, the eighth wonder of the world. See good/bad.

Definition: state of surprise
Antonyms: calmness, dullness, expectation, indifference, unexcitement

The noun has one meaning:

Meaning #1: the feeling that accompanies something extremely surprising
  Synonym: amazement