The Collaborative International Dictionary of English v.0.48:

Yug \Yug\, Yuga \Yu"ga\, n. [Skr. yuga an age, a yoke. See
   Yoke.] (Hindoo Cosmog.)
   Any one of the four ages, Krita, or Satya, Treta, Dwapara,
   and Kali, into which the Hindoos divide the duration or
   existence of the world.
   [1913 Webster]