There are 2 types of definitions available.
1) Upavāsa means going back from sin and leading a good life. All actions which are not good, must be relinquished. Those who observe Upavāsa should abstain from using flesh, Masūra (pulse), caṇaka (a kind of gram), Varaku (a kind of grain), green leaves prepared), honey, rice etc. and from contact with women. He should not wear flowers, ornaments, or fashionable dress; should not inhale fragrant smoke, and fragrance of any sort.
2) Upavāsa means Staying near the God, by engaging in Spiritual related practices, while abstaining from food
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Upavāsa might have been derived from the Rig Vedic word Upasad (उपसद्), indicating To sit near to or worship.
Bṛhadāraṇyaka Upaniṣad 6.3.1 says about Upasad-vrata, for acquiring wealth, as follows:
He who wishes to attain greatness (should perform) on an auspicious day in a fortnight in which the moon waxes, and under a male constellation, during the northward march of the sun, (a sacrifice in the following manner): He should undertake for twelve days a vow connected with the Upasads (i.e. *live on milk*), collect in a cup or bowl made of fig wood, all herbs and their grains, sweep and plaster (the ground), purify the offerings in the prescribed manner, interpose the Mantha (paste made of those things).
All herbs that are available and within one’s means: The ten cultivated species such as rice and barley, must be included.
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Interestingly, Ramayana says Upavāsa should be undertaken according to one's stamina.
उपवासः च कर्तव्या यथा प्राणेन मैथिलि |
जटा भारः च कर्तव्यो वल्कल अम्बर धारिणा || २-२८-१३
"Oh, Sita the princess of Mithila! Fasting is to be done according to one's stamina. Clothes of bark are to be worn and mass of matted hair has to be worn on the head."
