芥子劫 (Jpn keshi-ko )
A designation for the time period known as a kalpa, named for a way of illustrating its duration. According to the Miscellaneous Agama Sutra, a kalpa is longer than the time needed to remove all the mustard seeds filling a city one yojana (about 7 kilometers) square, if one takes away one seed every hundred years. A mustard-seed kalpa is also illustrated in The Treatise on the Great Perfection of Wisdom, in which the size of the city is described as forty ri square (one ri being about 450 meters). In Buddhist scriptures, the mustard seed is often used as a symbol for minuteness. For example, the "Devadatta" (twelfth) chapter of the Lotus Sutra says: "I (Bodhisattva Wisdom Accumulated) observe that, throughout the major world system, there is not a single spot tiny as a mustard seed where this bodhisattva [a previous incarnation of Shakyamuni Buddha) failed to sacrifice body and life for the sake of living beings. Only after he had done that was he able to complete the way to enlightenment."