From http://en.wikipedia.org/wiki/Meditation

Meditation is the practice of focusing the mind, often formalized into a specific routine. Meditation is usually recognized as a component of Eastern religions, originating in Vedic Hinduism. It encompasses any of a wide variety of spiritual practices which emphasize mental activity or quiescence. Meditation can also be used for personal development, such as the exercises of Hatha yoga

The English word meditation comes from the Latin meditatio, which originally indicated every type of physical or intellectual exercise, then later evolved into the more specific meaning "contemplation." This usage is found in Christian spirituality, for example, "meditations on the sufferings of Christ"; as well as Western philosophy, as in Descartes' Meditations on First Philosophy, a set of six mental exercises which systematically analyze the nature of reality.

However, "meditation" in its modern sense also refers to a separate practice unrelated to Christian meditations. In the late nineteenth century, Theosophists adopted the word "meditation" to refer to various spiritual practices drawn from Hinduism, Buddhism, and other Eastern religions. Thus the English word "meditation" does not exclusively translate any single term or concept, and can be used to translate words such as the Sanskrit dhyana, samadhi, or pranayama.

Meditation is usually defined as one of the following:
a state that is experienced when the mind dissolves and is free of all thoughts
focusing the mind on a single object (such as a religious statue, or one's breath, or a mantra)
a mental "opening up" to the divine, invoking the guidance of a higher power
reasoned analysis of religious teachings (such as impermanence, for Buddhists).

Its ritual and contemplative qualities are similar to prayer in Western religions, but prayer emphasizes communication with a higher being, whereas meditation focuses on developing oneself.

Meditation may be for a religious purpose, but even before being brought to the West it was used in secular contexts, such as the martial arts. Beginning with the Theosophists, though, meditation has been employed by a number of religious and spiritual movements, such as Hatha yoga and the New Age movement, as well as limited use in Christianity. It has been suggested that the recent popularity of "meditation" as a religious practice in the West signals some discomfort with more traditional Christian and Jewish practices such as prayer. Others see meditation and prayer as harmonious: Edgar Cayce taught that "Through prayer we speak to God. In meditation, God speaks to us."

From the point of view of psychology, meditation can induce an altered state of consciousness. However, many religious people would challenge the assumption that such mental states (or any other visible result) are the "goal" of meditation. The goals of meditation are varied, and range from spiritual enlightenment, to the transformation of attitudes, to better cardiovascular health.

According to Perez-De-Albeniz & Holmes (2000), the different techniques of meditation can be classified according to their focus. Some focus on the field or background perception and experience, also called mindfulness; others focus on a preselected specific object, and are called "'concentrative' meditation." There are also techniques that shift between the field and the object.

Categorizing the varieties of meditation is difficult. One common way is according to religion or lineage. But some meditative traditions, such as yoga or tantra, are common to several religions or occur outside religious contexts. Therefore, to avoid controversy, this article will not attempt to classify all meditations into a religious class or lineage.

Meditation has always enjoyed a central place within Buddhism. The Buddha himself was said to have achieved enlightenment while meditating under a Bodhi tree. Most forms of Buddhism distinguish between samatha and vipassana meditation, both of which are necessary for enlightenment. The former consists of learning to focus the attention single-pointedly; the latter involves seeing the true nature of reality.

Theravada Buddhism emphasizes vipassana meditation directed towards anapana, mettā bhāvanā, or 38 other traditional topics .

In Japanese Mahayana schools, Tendai (Tien-tai), concentration is cultivated through highly structured ritual. Especially in the Chinese Chan Buddhism school (which branched out into the Japanese Zen, and Korean Seon schools), ts'o ch'an meditation and koan meditation practices are extremely important, allowing a practitioner to directly experience the true nature of reality (each of the names of these schools derives from the sanskrit dhyana, and translates into "meditation" in their respective languages). The esoteric Shingon sect shares many features with Tibetan Buddhism.

Tibetan Buddhism emphasizes tantra for its senior practitioners; hence its alternate name of Vajrayana Buddhism. However, visitors to Tibetan monasteries are often surprised to discover that many monks go through their day without "meditating" in a recognizable form, but are more likely to chant or participate in group liturgy

Christian traditions have various practices which might be identified as forms of "meditation." Many of these are monastic practices. Some types of Christian prayer, such as the rosary, Adoration (focusing on the eucharist) in Catholicism or the hesychasm in Eastern Orthodoxy, may be compared to the form of Eastern meditation that focuses on an individual object, but these comparisons often overlook the importance of prayer in these rituals.