The Collaborative International Dictionary of English v.0.48:

Thammuz \Tham"muz\, Tammuz \Tam"muz\, n. [Heb. thamm[=u]z.]
   1. A deity among the ancient Syrians, in honor of whom the
      Hebrew idolatresses held an annual lamentation. This deity
      has been conjectured to be the same with the Phoenician
      Adon, or Adonis. --Milton.
      [1913 Webster]

   2. The fourth month of the Jewish ecclesiastical year, --
      supposed to correspond nearly with our month of July.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Thammuz
    n 1: the tenth month of the civil year; the fourth month of the
         ecclesiastic year (in June and July) [syn: Tammuz,
         Thammuz]