Symbol for satori with moon nimitta or meditative counterpart sign (soniei.com)
Satori is typically juxtaposed with a related term known as kensho, which translates as "seeing one's nature." Kensho experiences tend to be briefer glimpses, while satori is considered a deeper, intuitive spiritual experience. Satori has been described as being similar to awakening one day with an additional pair of arms and only later learning how to use them.
Practitioners of Zen attempt to have this personal experience. The traditional way of attaining it, and the typical way it is taught to Zen students in the West, is through the use of koans such as those found in the collection known as the Gateless Gate (the Mumonkan). Koans are rhetorical riddles/puzzles students use to assist in their realization. They are stories, words, and phrases used by early Zen masters.
The Gateless Gate was assembled by the early 13th century Chinese Zen master Wumen Hui-k'ai. Zen master Yuelin Shiguan (Japanese, Gatsurin Shikan, 1143-1217) gave Wumen the koan Zhaozhou’s Dog, which Wumen struggled with for six years before he finally attained realization. After his understanding had been confirmed by Yuelin, Wumen wrote the following enlightenment poem:
A thunderclap under the clear blue sky
All beings on earth open their eyes;
Everything under heaven bows together;
Mount Sumeru leaps up and dances.
While kensho is sometimes used interchangeably with satori, kensho only refers to the first perception of one's Buddha-nature, often referred to as a first "awakening." Distinct from kensho, which is a clear but transitory glimpse of the true nature of existence, satori refers to a deep or lasting realization.
Satori in the Zen tradition does not actually "happen" to an individual. Rather, it is a realization out of all concepts including the individual. Practitioners of Zen Buddhism, however, work to realize the true nature of existence. The student's mind must often be prepared by rigorous study, koans, and meditation to clear the mind of all attachments to the sensual world.
It is therefore customary to use the word satori, rather than kensho, when referring to the realization of the Buddha, patriarchs, and bodhisattvas. These figures recognized that "all things are Buddha things" and therefore any separation between self and the universe is illusory.
According to D. T. Suzuki, "Satori is the raison d'être of Zen, without which Zen is no Zen. Therefore every contrivance, disciplinary and doctrinal, is directed towards satori."