Analysis of information sources in references of the Wikipedia article "Yuga" in English language version.
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 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.
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).
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.
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.
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.
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).
