Yuga (Devanāgari: युग) in Hindu philosophy is the name of an 'epoch' or 'era' within a cycle of four ages. These are the Satya Yuga, the Treta Yuga, the Dvapara Yuga, and finally the Kali Yuga. According to Hindu cosmology, life in the universe is created, destroyed once every 4.1 to 8.2 billion years, which is one full day (day and night) for Brahma. The lifetime of a Brahma himself may be 311 trillion and 40 billion years. The cycles are said to repeat like the seasons, waxing and waning within a greater time-cycle of the creation and destruction of the universe. Like Summer, Spring, Winter and Autumn, each yuga involves stages or gradual changes which the earth and the consciousness of mankind goes through as a whole. A complete yuga cycle from a high Golden Age of enlightenment to a Dark Age and back again is said to be caused by the solar system's motion around another star.
Read more about Yuga: Kalki, Durations of Four Yugas, Sri Yukteswar's Teachings On The Yugas, The Four Yugas and Brahmajnana