kalpa ［劫］ (; kō): In ancient Indian cosmology, an extremely long period of time. There are various views on the length of a kalpa. According to The Treatise on the Great Perfection of Wisdom, a kalpa is longer than the time required to wear away a cube of rock forty ri (one ri being about 450 meters) on each side, by brushing it with a piece of cloth once every hundred years. Great Perfection of Wisdom also defines a kalpa as being longer than the time needed to remove all the mustard seeds filling a city forty ri square, if one takes away one seed every hundred years. Nearly identical explanations appear in the Miscellaneous Āgama Sutra, where the length of each side of the rock is given as one yojana (about 7 kilometers), and the size of the city as one yojana square.
The word kalpa is also used in describing the formation and disintegration of the world. According to Buddhist cosmology, a world perpetually repeats a four-stage cycle of formation, continuance, decline, and disintegration. The periods corresponding to these four stages are called the four kalpas. Each of these four kalpas—the kalpa of formation, the kalpa of continuance, the kalpa of decline, and the kalpa of disintegration—lasts for twenty small kalpas. A small kalpa is defined in terms of cyclical changes said to occur repeatedly in the human life span during the kalpa of continuance. Over the course of a small kalpa, the human life span increases from 10 to 80,000 years and then decreases from 80,000 to 10 years. The increase of life span occurs at the rate of one year every hundred years, and the decrease of life span also occurs in the same way. During the kalpa of continuance, a world and its inhabitants continue to exist for twenty small kalpas, that is, while the human life span repeats its increase and decrease in this way. The time required for the life span to increase from 10 to 80,000 years is 79,990 years multiplied by 100, which equals 7,999,000 years. Exactly the same number of years is necessary for the decrease in life span from 80,000 to 10 years; that is, 7,999,000 is multiplied by two, equaling 15,998,000 years. Thus, this number represents the length of a small kalpa. Because a small kalpa is often described simply as a kalpa, 15,998,000 years, or about 16,000,000 years, is often given as the definition of the length of a kalpa.