In the Rigveda, a yuga refers to generations, a long period, a very brief period, 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.
The names "Yuga" and "Age" commonly denote a catur-yuga (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.[a]
Yuga (Sanskrit: 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, yugānāṃ, and yuge, derived from yuj (Sanskrit: युज्, lit. 'to join or yoke'), believed derived from *yeug- (Proto-Indo-European: lit. 'to join or unite').
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'.
The word yuga occurs at least thirty-eight times in the Rigveda, but the meaning is rather doubtful. In a few places yuga means yoke ... In many places it appears to refer to a very brief period ... Generally yuga appears to mean in the Rigveda 'generation' (lessening the life of human generations) ... In other places 'yuga' must be given the sense of a 'long period of time' ...
The cycle [of creation and destruction] is either called a yuga (MBh. 1.1.28; 12.327.89; 13.135.11), a kalpa, meaning a formation or a creation (MBh. 6.31.7 [= BhG. 9.7]; 12.326.70; 12.327.23), or a day of the brahman, or of Brahmā, the creator god (MBh. 12.224.28–31). Sometimes, it is simply referred to as the process of creation and destruction (saṃhāravikṣepa; MBh. 12.271.30, 40, 43, 47–49).
The period of 4,320,000 years is ordinarily styled Great Age (mahayuga), or, as above in two instances [1.15-16], Quadruple Age (caturyuga). In the Surya-Siddhanta, however, the former term is not once found, and the latter occurs only in these verses; elsewhere, Age (yuga) alone is employed to denote it, and always denotes it, unless expressly limited by the name of the Golden (krta) Age.
catur-yuga-sahasraṁ tu brahmaṇo dinam ucyate ।
sa kalpo yatra manavaś caturdaśa viśām-pate ॥ 2 ॥
(2) 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.
sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ ।
rātriṁ yuga-sahasrāntāṁ te 'ho-rātra-vido janāḥ ॥ 17 ॥
(17) 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.
Jugo: in general, to join, connect. Jugum: [kindred to sanskrit yuga from yug-, jungere; v. jungo], a yoke for oxen, a collar for horses.