Kensho, literally "seeing the nature", is an experience described in the context of Zen Buddhism. The term is often used to denote an initial awakening experience, seeing one's True-Nature or Buddha-Nature, that can be enlarged and clarified through further practice in daily life.
The Kensho experience
In Kensho, one experiences the illusionary nature of the separate self ('I'). Because of the nature of the mind, any perception seems to involve a perceived object, the process of perception, and a perceiving subject.
For example, 'I see you': I - the subject, see - the process of perception, you - the object, that appears to be separate from the perceived objects. Trying to find the 'I', the subject, through introspection leads to the realisation that this 'I', is completely dependent on the process of perception and associated thought/feeling complex and the memories tied to them.
Working towards this realisation is usually a lengthy process of meditation and introspection under guidance of a Zen or other Buddhist teacher. The method is known as: 'Who am I', since it is this question that guides the enquiry into one's true nature. The realization that there is no 'I' that is doing the thinking, but rather that the thinking process brings forth the illusion of an 'I', is a step on the way to Kensho.
Satori is a Zen Buddhist term for enlightenment. The word literally means 'to understand'. It is sometimes loosely used interchangeably with Kensho, but Kensho refers to the first perception of the Buddha-Nature or True-Nature. The kensho experience may not hold as further training is still necessary by the Monk or Lay. Satori on the other hand refers to the lasting experience. Think of when a baby first walks, after much effort, it stands upright, find its balance and walks a few steps, then falls (Kensho). After continued effort the child will one day find that it is able to walk all the time (Satori).
Once the True-Nature has been seen it is customary to use Satori when referring to the enlightenment of the Buddha and the Patriarchs, as their enlightenment was permanent.
The Zen Buddhist experience commonly recognizes enlightenment as a transitory thing in life, almost synonymous with the English term epiphany, and Satori is the realization of a state of epiphanic enlightenment. Because all things are transitory according to Zen philosophy, however, the transitory nature of Satori is not regarded as limiting in the way that a transitory epiphany would be in Western understandings of enlightenment.
The transitory nature of Satori, as opposed to the more enduring Nirvana that is sought in the Buddhist traditions of India, owes much to Taoist influences on Chán Buddhism in China, from which Zen Buddhism of Japan evolved. Taoism is a mystical philosophy that emphasizes the purity of the moment, whereas the Hindu roots of Indian Buddhism lend a longer view toward escaping the Karmic prison of perpetual reincarnation in the material world. From Taoism's attention to the importance of the moment and Mahayana Buddhism's almost nihilistic denial of the validity of individual existence, Zen Buddhism with its concept of the transitory state of Satori was born.