Satori is the spiritual goal of Zen Buddhism (in Chinese: wu). It is a key concept in Zen. Whether it comes to you suddenly seemingly out of nowhere as found in the Enlightenment process called Aparka Marg, or after an undetermined passage of time centered around years of intense study and meditation as with the female Zen adept Chiyono, or after forty unrelenting years as with the Buddha's brother Ananda, there can be no Zen without that which has come to be called Satori. As long as there is Satori, then Zen will continue to exist in the world.
Satori roughly translates into individual Enlightenment, or a flash of sudden awareness. Satori is as well an intuitive experience. The feeling of Satori is that of infinite space. A brief experience of Enlightenment is sometimes called Kensho. Semantically, Kensho and Satori have virtually the same meaning and are often used interchangeably. In describing the Enlightenment of the Patriarchs, however, it is customary to use the word Satori rather than Kensho, the term Satori implying a deeper experience. The level of Enlightenment reached by the Buddha and others of similar ilk is refered to as Anuttara Samyak Sambodhi.
There are, as seen in the above, more than one "level" of Self-realization. Most levels, except perhaps Anuttara Samyak Sambodhi, have been blanketed with what has become now a more general term, "Satori," Satori having fallen into the day-to-day lexicon exemplified in a variety of sources from the Eight Jhana States, to the Five Degrees of Tozan, to the Five Varieties of Zen. There are also, as claimed by some, three kinds, levels or varieties of Satori --- typically listed as being 1) emotion-based or Mystical Satori, 2) mind-based or Intellectual Satori, and 3) desire-based or Cosmic Satori.
It was not always that way. If you scroll down to the Satori discription by D.T. Suzuki, below, you will gain a much greater insight into the original meaning of Satori. There is an enormous difference between say something like a rather uncomplicated early stage such as as Laya to the somewhat deeper initial step of Inka Shomei and the state of Enlightenment at the level of the Buddha.
The only way that one can "attain" Satori is through personal experience. The traditional way of achieving Satori, and the most typical way taught to Zen students in the west --- but NOT the only way --- is through the use of Koans such as those found in the Gateless Gate, the Mumonkan. Koans are "riddles" students use to assist in the realization of Satori; these words and phrases were also used by the early Zen masters. See Regarding Mu.
Another method is meditation. Satori can be brought about through Zazen meditation. This meditation would create an objective self associated awareness with a feeling of joy that overrides any other feelings of joy or sorrow.
Even though Satori is a key concept in Zen, it should be brought to the attention of the reader that Zen and it's traditions does NOT have exclusive rights to the Enlightenment experience. That which is called Satori in Zen is a term that is wrapped around a phenomenon that "IS" and that "IS" is not "owned" by any group, religion, or sect.
Many, many, occurrences of that particular "phenomenon" has transpired both inside and outside the Doctrine of Buddhism. The person who was to become the Sixth Patriarch in the Chinese Lineage of Ch'an was Enlightened as a young boy when he overheard a sentence being spoken from the Diamond Cutter Sutra. He had gone into town to sell firewood for his mother when he just happened to hear the line. Until that point in time he had not received any formal practice in meditation, nor was he versed in Buddhism to any great extent, if at all. So too, again outside the scriptures, the great Indian sage Bhagavan Sri Ramana Maharshi was a typical of his culture teenage boy and most certainly not deeply seeped into formal religious tracts, when all of a sudden out of the blue, Satori-like, he was Awakened to the Absolute.
It is often said that when you truly need a teacher --- or that which will function in lieu of a teacher --- that is, a teacher or Satori for example, will fall upon you. This may due to some inexplicable serendipity. It may be due to the fact that the seeker has searched deeply within himself or herself and determined what sort of instruction seems to be required. It could be swept over him or her like the First Death Experience of the Bhagavan Sri Ramana Maharshi, or the Bhagavan's little known Second Death Experience. Or it could be a spiritual desperation on the part of the seeker, or maybe no more than a successful sales pitch by a teacher (sincere or not). It may be a combination of the previous factors, or some intuitive awareness beyond expression. For whatever the reason, the saying often applies and the coming together of the results of inner and outside forces, some within one's control, some without.
However, in the end , it is NOT just potential Zen masters in ancient China nor people in India that such events transpire, but everyday people as well. There are numerous Awakening Experiences in the Modern Era, but, even if those experiences parallel that which is called Satori, those experiences are not always called Satori.