The Collaborative International Dictionary of English v.0.48:

Tuning \Tun"ing\ (t[=u]n"[i^]ng),
   a. & n. from Tune, v.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tuning fork
    n 1: a metal implement with two prongs that gives a fixed tone
         when struck; used to tune musical instruments