Yuga in Dharmic 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.
चत्वारि भारते वर्षे युगानि भरतर्षभ
कृतं तरेता दवापरं च पुष्यं च कुरुवर्धन
पूर्वं कृतयुगं नाम ततस तरेतायुगं विभॊ
संक्षेपाद दवापरस्याथ तथ पुष्यं परवर्तते
चत्वारि च सहस्राणि वर्षाणां कुरुसत्तम
आयुः संख्या कृतयुगे संख्याता राजसत्तम
तत्र तरीणि सहस्राणि तरेतायां मनुजाधिप
दविसहस्रं दवापरे तु शते तिष्ठति संप्रति
न परमाण सथितिर हय अस्ति पुष्ये ऽसमिन भरतर्षभ
गर्भस्थाश च मरियन्ते ऽतर तथा जाता मरियन्ति च
Those who understand the cosmic day and cosmic night know that one day of creation is a thousand cycles, and that the night is of equal length. At the dawning of that day all objects in manifestation stream forth from the Unmanifest, and when evening falls they are dissolved into It again. The same multitude of beings, which have lived on earth so often, all are dissolved as the night of the universe approaches, to issue forth anew when morning breaks. Thus is it ordained. In truth, therefore, there is the Eternal Unmanifest, which is beyond and above the Unmanifest Spirit of Creation, which is never destroyed when all these being perish. The wise say that the Unmanifest and Indestructible is the highest goal of all; when once That is reached, there is no return. That is My Blessed Home. – Bhagvad Gita, 8.17
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.Each yuga involves stages or gradual changes which the earth and the consciousness of mankind goes through as a whole.