Source : Webster's Revised Unabridged Dictionary (1913)

Aposiopesis \Ap`o*si`o*pe"sis\ (?; 277), n. [L., fr. Gr. ?, from
   ? to be quite silent.] (Rhet.)
   A figure of speech in which the speaker breaks off suddenly,
   as if unwilling or unable to state what was in his mind; as,
   ``I declare to you that his conduct -- but I can not speak of
   that, here.''

Source : WordNet®

aposiopesis
     n : breaking off in the middle of a sentence (as by writers of
         realistic conversations)
     [also: {aposiopeses} (pl)]