Yuga

yuga(s) A yuga is a great cycle of time - one of the four ages, or stages, of cosmic history: the Satya Yuga, or Golden Age; the Tretya Yuga, or Silver Age; the Dwapara Yuga, or Copper (or Bronze) Age; and the Kali Yuga, or Iron Age.

These four ages together comprise a mahayuga (also known as a kalpa or manvantara).

For more, see Four Cycles of Life.