Yuga (Sanskrit) Age; an age of the world, of which there are four -- satya yuga, treta yuga, dvapara yuga, and kali yuga -- which proceed in succession during the manvantaric cycle. Each yuga is preceded by a period called in the Puranas, sandhya (twilight, transition period, dawn) and followed by another period of like duration often called sandhyansa (a portion of twilight). Each of these transition periods is one-tenth of its yuga. The group of four yugas is first computed by the divine years or years of the gods -- each such year being equal to 360 years of mortal men.
Thus we have, in divine years:
1. Krita or Satya Yuga . . 4,000
Sandhya . . . . . . . . 400
Sandhyansa . . . . . . 400
4,800 or 1,728,000 mortal years
2. Treta Yuga . . . . . . . 3,000
Sandhya . . . . . . . . 300