The Collaborative International Dictionary of English v.0.48:

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."
   [1913 Webster]

WordNet (r) 3.0 (2006):

aposiopesis
    n 1: breaking off in the middle of a sentence (as by writers of
         realistic conversations)