The Four World-Ages

The yugas are the four world-ages of Hindu cosmology, Satya (or Krita), Treta, Dvapara and Kali, which run in sequence, each shorter and more degenerate than the last, before the cycle begins again. Satya Yuga is an age of truth and virtue; each following age sees dharma decline, until Kali Yuga, the present age, the shortest and darkest, after which the cycle renews.

The lengths given in the Puranas are enormous and vary between texts, reaching into hundreds of thousands of years, with the four together making a mahayuga and a thousand mahayugas making a kalpa, a day of Brahma.