Overview
A kalpa is a long period of time (aeon) in Hindu and Buddhist cosmology, generally between the creation and recreation of a world or universe.
Etymology
Kalpa in this context, means "a long period of time (aeon) related to the lifetime of the universe (creation)." It is derived from कॢप् (kḷp) + -अ (-a, nominalizing suffix).
Hinduism
In Hinduism, a kalpa is equal to 4.32 billion years, a "day of Brahma" (12-hour day proper) or one thousand mahayugas, measuring the duration of the world. Each kalpa is divided into 14 manvantara periods, each lasting 71 Yuga Cycles (306,720,000 years). Preceding the first and following each manvantara period is a juncture (sandhya) equal to the length of a Satya Yuga (1,728,000 years). A kalpa is followed by a pralaya (dissolution) of equal length, which together constitute a day and night of Brahma. A month of Brahma contains thirty such days and nights, or 259.2 billion years. According to the Mahabharata, 12 months of Brahma (=360 days) constitute his year, and 100 such years his life called a maha-kalpa (311.04 trillion years or 36,000 kalpa + 36,000 pralaya). Fifty years of Brahma are supposed to have elapsed, and we are now in the Shveta-Varaha Kalpa or the first day of his fifty-first year. At the end of a kalpa, the world is annihilated by fire.
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