Meditation encompasses an extremely broad array of practices connected to many of the world's religious and philosophical traditions. Different traditions and even personal interpretations may and do affect the definition of the word, making it difficult to reach some consensus on what exactly Meditation is or what its goals are. For that reason, much of this article refers to specific traditions.
Meditation generally includes avoiding (though not harshly) random thought processes and fantasies, and a calming and focusing of the mind. It is not effortful, and can be experienced as just happening. Different practices involve focusing one's attention differently, and a variety of positions and postures including sitting cross-legged, standing, laying down, and walking (sometimes along designated floor patterns).
The stated purpose of meditation varies almost as much as the practices. It has been seen as a means of gaining experiential insight into the nature of reality (religious/spiritual or not), or communing with the Deity/Ultimate Reality. Even without the spiritual aspects, many have gained concentration, awareness, self-discipline and equanimity.
In the Samadhi or Shamatha, or concentrative, techniques of meditation, the mind is kept closely focused on a particular word, image, sound, person, or idea. This form of meditation is found in Buddhist and Hindu traditions including Yoga, in Medieval Christianity, Jewish Kabbalah, and in some modern metaphysical schools. Related to this method is the method developed by Eknath Easwaran. He called it "passage meditation" -- silent repetition in the mind of memorized inspirational passages from the world's great religions. As Easwaran says, "The slow, sustained concentration on these passages drives them deep into our minds; and whatever we drive deep into consciousness, that we become."
In Vipassana (insight, or seeing things as they are) meditation the mind is trained to notice each perception or thought that passes, but without "stopping" on any one. This is a characteristic form of meditation in Buddhism, especially in some Theravada traditions, and is also a component of Zazen, the term for meditation practice in Zen. In at least some forms of vipassana, you do not attend to whatever perceptions arise, but purposely move your attention over your body part by part, checking for perceptions, being aware and equanimous with them, and moving on. This form of meditation has some resemblance with "choiceless awareness" the kind of meditation that Jiddu Krishnamurti talked about.
In anapanasati meditation attention is focused on the breath. Vipassana and anapanasati are parts of broader notion of mindful awareness, which is part of the eightfold path in Buddhism, and expounded upon the in the Satipatthana sutta.
Health Uses and Benefits
Meditation has entered the mainstream of health care as a method of stress and pain reduction. For example, in an early study in 1972 transcendental meditation was shown to effect the human metabolism by lowering the biochemical byproducts of stress, such as lactate (lactic acid), and by decreasing heart rate and blood pressure and inducing favorable brain waves. (Scientific American 226: 84-90 (1972))
As a method of stress reduction, meditation is often used in hospitals in cases of chronic or terminal illness to reduce complications associated with increased stress including a depressed immune system. There is a growing consensus in the medical community that mental factors such as stress significantly contribute to a lack of physical health, and there is a growing movement in mainstream science to fund and do research in this area (e.g. the establishment by the NIH in the U.S. of 5 research centers to research the mind-body aspects of disease.)
Dr. James Austin, a neurophysiologist at the University of Colorado, reported that Zazen or Zen meditation rewires the circuitry of the brain in his landmark book Zen and the Brain. This has been confirmed using sophisticated imaging techniques which examine the electrical activity of the brain.
Dr. Herbert Benson of the Mind-Body Medical Institute, which is affliated with Harvard and several Boston hospitals, reports that meditation induces a host of biochemical and physical changes in the body collectively refered to as the "relaxation response." The relaxtion response includes changes in metabolism, heart rate, respiration, blood pressure, and brain chemistry.