Kalpa (aeon)
Kalpa is a Sanskrit word (कल्प kalpa) meaning an aeon, or a relatively long period of time (by human calculation) in Hindu and Buddhist cosmology. The concept is first mentioned in the Mahabharata. The definition of a kalpa equalling 4.32 billion years is found in the Puranas (specifically Vishnu Purana and Bhagavata Purana).
Read more about Kalpa (aeon): Buddhism, Hinduism, Kalpa and Other Periods of Time, Names of The Kalpas, Brahma Kumaris