The archaic form of the Sanskrit word "yuga" is "yug". Other forms are "yugam", "yugānāṃ" and "yuge". In Latin, "juga" or "jug" are used from "jugum" meaning "yoke", used to connect two oxen (e.g. cali-juga = kali-yuga). The word "yuga", as well as "yoga", are derived from Sanskrit: युज्, romanized: yuj, lit. 'to join or yoke', believed to be derived from Proto-Indo-European language: yeug, 'to join or unite'.
There are a total of four yugas : Satya Yuga, Treta Yuga,Dvapara Yuga, andKali Yuga, each having 1/4th less duration and dharma than the previous (Satya most, Kali least). The descending yugas see a gradual decline of dharma, wisdom, knowledge, intellectual capability, lifespan, emotional and physical strength.
Within a yuga are Sandhis, or a starting Sandhya and ending Sandhyansa, both lasting 1/10th the main part of the yuga. Most of the characteristic changes can occur in these Sandhyas and Sandhyansas, especially if that change is from Kali Yuga to Satya Yuga (two extremes).
Satya Yuga (Krita Yuga, "the age of truth", or "hindu golden age") : the first and best yuga. It is the age of truth and perfection. This yuga has no crime, and all humans are kind and friendly. The Krita Yuga is so named because there is one religion, and all men are saintly : therefore they are not required to perform religious ceremonies. Humans are long living, powerfully built, honest, youthful, vigorous, erudite and virtuous. The Vedas are one. All mankind can attain to supreme blessedness. There is no agriculture or mining, as the earth yields those riches on its own. Weather is pleasant, and everyone is happy. There is no religious sect. There is no disease, decrepitude, or fear of anything. Virtue reigns supreme. Human stature is 21 cubits (33 ft, 6 inches). Average human lifespan is 100,000 years.
Treta Yuga : this is the second yuga in chronological order. However, "treta" means the "third". In this age, virtue diminishes slightly. At the beginning of the age, many emperors rise to dominance and conquer the world. Wars become frequent and weather begins to change to extremities. People become slightly diminished, compared to their predecessors. Agriculture, labour and mining become existent. There are 3 quarter virtues and 1 quarter sin. Normal human stature is 14 cubits (22 ft, 4 inches). Average human lifespan is 10,000 years.
Dvapara Yuga : this is the third yuga in order. However, "dvapara" means "two"/"second". In this age, people become tainted with qualities, and aren't as strong as their ancestors. Diseases become rampant. Humans are discontent and fight each other. Vedas are divided into four parts. People still possess characteristics of youth in old age. Average lifespan of humans is around a few centuries. There are 1 half virtue and 1 half sin. Normal human stature is 7 cubits (11 ft, 2 inches). Average human lifespan is 1,000 years.
Kali Yuga : the final age. It is the age of darkness and ignorance. People stop following dharma, and lack virtue. They become slaves to their passions and are barely as powerful as their earliest ancestors in the Satya Yuga. Society falls into disuse and people become liars and hypocrites. Knowledge is lost and scriptures are diminished. Humans eat forbidden and dirty food. The environment is polluted, water and food become scarce. Wealth is heavily diminished. Families become non-existent. There is 1 quarter virtue, and 3 quarter sins. Normal human stature is 3.5 cubits (5 ft, 3 inches). Average human lifespan is 100 years.
^Sundarraj, M. (1997) [1st ed. 1994]. "Ch. 4 Asvins—Time-Keepers". In Mahalingam, Dr. N. (ed.). RG Vedic Studies. Coimbatore: Rukmani Offset Press. p. 219. It is quite clear that the smallest unit was the 'nimisah' ['winking of eyes'], and that time in the general sense of past, present and future was indicated by the word 'yuga'.
^A kalpa is described as lasting 1,000 catur-yuga in Bhagavata Purana 12.4.2 ("catur-yuga") and Bhagavad Gita 8.17 ("yuga"): * "Śrīmad-Bhāgavatam (Bhāgavata Purāṇa) 12.4.2". Bhaktivedanta Vedabase. Retrieved 2020-05-10. One thousand cycles of four ages [catur-yuga] constitute a single day of Brahmā, known as a kalpa. In that period, O King, fourteen Manus come and go. * "Bhagavad-gītā As It Is 8.17". Bhaktivedanta Vedabase. Retrieved 2020-05-10. By human calculation, a thousand ages [yuga] taken together form the duration of Brahmā’s one day. And such also is the duration of his night.