Source : Webster's Revised Unabridged Dictionary (1913)

Tuning \Tun"ing\,
   a. & n. from {Tune}, v.

   {Tuning fork} (Mus.), a steel instrument consisting of two
      prongs and a handle, which, being struck, gives a certain
      fixed tone. It is used for tuning instruments, or for
      ascertaining the pitch of tunes.

Source : WordNet®

tuning fork
     n : a metal implement with two prongs that gives a fixed tone
         when struck; used to tune musical instruments