Private religious devotion or mental exercise, in which techniques of concentration and contemplation are used to reach a heightened level of spiritual awareness. The practice has existed in all religions since ancient times. In Hinduism it has been systematized in the school of Yoga. One aspect of Yoga, dhyana (Sanskrit: concentrated meditation), gave rise to a school of its own among the Buddhists, becoming the basis of Zen. In many religions, meditation involves verbal or mental repetition of a single syllable, word, or text (e.g., a mantra). Visual images (e.g., a mandala) or mechanical devices such as prayer wheels or rosaries can be useful in focusing concentration. In the 20th century, movements such as Transcendental Meditation emerged to teach meditation techniques outside a religious context.