The Encyclopædia Britannica 2003 Ultimate Reference Suite CD-ROM, in an article on Meditation, says:

The practice of meditation has occurred worldwide since ancient times in a variety of contexts. It may serve purely quietistic aims, as in the case of certain reclusive mystics; it may be viewed as spiritually or physically restorative and enriching to daily life, as in the case of numerous religious orders and the majority of secular practitioners; or it may serve as special, potent preparation for a particular, usually physically or otherwise strenuous activity, as in the case of the warrior before battle or the musician before performance. In recent medical and psychological studies, meditational techniques have proved effective in skilled practitioners in controlling pulse and respiratory rates and effective to varying degrees in the symptomatic control of migraine headache, hypertension, and hemophilia, among other conditions.

Meditation in some form has been systematized in most great religions of the world. The Hindu philosophical school of Yoga prescribes a highly elaborated process for the purification of body, mind, and soul. One aspect of Yoga practice, dhyana (Sanskrit: “concentrated meditation”), became the focus of a school of its own among the Buddhists, in China as Ch'an and, subsequently, in Japan as Zen. In numerous religions, spiritual purification may be sought through the verbal or mental repetition of a prescribed efficacious syllable, word, or text (e.g., the Hindu and Buddhist mantra; Islamic dhikr; Christian Jesus prayer). The focusing of attention upon a visual image (e.g., a flower, a distant mountain) is a common technique in informal contemplative practice and has been formalized in several traditions. Tantric Buddhists of Tibet, for example, regard the mandala (Sanskrit: “circle”) diagram as a collection point of universal forces, accessible to man by meditation. Tactile and mechanical devices, such as the rosary and the prayer wheel, and music play a highly ritualized role in many contemplative traditions.