The Collaborative International Dictionary of English v.0.48:

Vendetta \Ven*det"ta\, n. [It.]
   1. A blood feud; private revenge for the murder of a kinsman.
      [1913 Webster]

   2. Any feud or contention that is bitter and prolonged;
      however, the deep enmity may be held by only one party to
      the dispute; as, the former Mayor nurtured a lifelong
      vendetta against the candidate who defeated him.
      [PJC]

WordNet (r) 3.0 (2006):

vendetta
    n 1: a feud in which members of the opposing parties murder each
         other [syn: vendetta, blood feud]