Source : Webster's Revised Unabridged Dictionary (1913)

Speechless \Speech"less\, a.
   1. Destitute or deprived of the faculty of speech.

   2. Not speaking for a time; dumb; mute; silent.

            Speechless with wonder, and half dead with fear.
                                                  --Addison.
      -- {Speech"less*ly}, adv. -- {Speech"less*ness}, n.

Source : WordNet®

speechlessness
     n : the property of being speechless