(skt.: kalpa/yuga; tib.: kal pa) In ancient Indian cosmology, an extremely long period of time.
In general, a small kalpa is represented as 16,800,000 years, a kalpa as 336,000,000 years and a mahakalpa is 1,334,000,000 years. There is also another kalpa, even longer than mahakalpa, which is called Countless eon. This is the time it takes after you decide to begin collecting virtue to become a Buddha to actually become a Buddha. Master Vasubandhu says in his Abhidharmakosha that Countless eon is a period of three countless (countless actually means a number: about ten to the sixtieth power) mahakalpas.