Mantra

Mantra (Sanskrit) means a sacred utterance, numinous sound, or a syllable, word, phonemes, or group of words believed by some to have psychological and spiritual power. Mantra may or may not be syntactic or have literal meaning; the spiritual value of mantra comes when it is audible, visible, or present in thought.

Earliest mantras were composed in Vedic times by Hindus in India, and those are at least 3000 years old. Mantras are now found in various schools of Hinduism, Buddhism, Jainism and Sikhism. Similar hymns, chants, compositions and concepts are found in Zoroastrianism, Taoism, Christianity and elsewhere.

The use, structure, function, importance and types of mantras vary according to the school and philosophy of Hinduism and of Buddhism. Mantras serve a central role in the tantric school of Hinduism. In this school, mantras are considered equivalent to deities, a sacred formula and deeply personal ritual, and considered to be effective only after initiation. However, in other schools of Hinduism, Buddhism, Jainism or Sikhism, this is not so.

Mantras come in many forms, including ṛc (verses from Rigveda for example) and sāman (musical chants from the Sāmaveda for example). They are typically melodic, mathematically structured meters, resonant with numinous qualities. At its simplest, the word (Aum, Om) serves as a mantra. In more sophisticated forms, they are melodic phrases with spiritual interpretations such as human longing for truth, reality, light, immortality, peace, love, knowledge and action.  In other forms, they are literally meaningless, yet musically uplifting and spiritually meaningful.

http://en.wikipedia.org/wiki/Mantra