Yuga

A yuga, in Hinduism, is generally used to indicate an age of time.

In the Rigveda, a yuga refers to generations, a period of time (whether long or short), or a yoke (joining of two things). In the Mahabharata, the words yuga and kalpa (a day of Brahma) are used interchangeably to describe the cycle of creation and destruction.

In post-Vedic texts, the words "yuga" and "age" commonly denote a Yuga Cycle (pronounced chatur yuga), a cycle of four world ages—for example, in the Surya Siddhanta and Bhagavad Gita (part of the Mahabharata)—unless expressly limited by the name of one of its minor ages: Krita (Satya) Yuga, Treta Yuga, Dvapara Yuga, or Kali Yuga.

Etymology
Yuga (युग) means "a yoke" (joining of two things), "generations", or "a period of time" such as an age, where its archaic spelling is yug, with other forms of yugam,, and yuge, derived from yuj (युज्), believed derived from  (Proto-Indo-European: 'to join or unite').

Meanings
The term "yuga" has multiple meanings, including representing the number 4 and various periods of time. In early Indian astronomy, it referred to a five-year cycle starting with the conjunction of the sun and moon in the autumnal equinox. More commonly, "yuga" is used in the context of kalpas, composed of four yugas. According to the Manusmriti, a kalpa starts with a Satya Yuga (4,000 years), followed by a Treta Yuga (3,000 years), a Dvapara Yuga (2,000 years), and ends with a Kali Yuga (1,000 years). According to Vishnu Purana, each Mahayuga comprises a Satya Yuga (1,728,000 human years), a Treta Yuga (1,296,000 years), a Dvapara Yuga (864,000 years), and a Kali Yuga (432,000 years).