A mantra is a repetitive group of words, that becomes a prolonged chant. Sometimes spoken and sometimes sung. Sometimes the words have meaning - calling upon a deity for instance - and sometimes they are just a group of words to focus the mind on a topic, such as health or happiness. A well known chant is that of Tibetan Buddhism 'Aum Mani Padme Hum' or 'Om Namashi Vaya' a Hindu chant. The repetitive words of many Christian prayers are also a chant, focussing the mind on devotion and clearing the mind of other thoughts.
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