Mantras ritual verbal formulas characteristic of South Asian religious traditions since Vedic times (c 1500 BC). The most likely etymology of the Sanskrit term mantra is 'an instrument (-tr) of thought (man-)'. Originally, a mantra designated any portion of the Vedic hymns, especially as used in rituals. In tantricism, beginning in the first century AD, mantras only partly resembling the Vedic type became the central component of ritual practice. Accordingly, Tantric Hinduism and Buddhism are often called, respectively, the science of mantra (mantra-shastra) and the way of mantra (mantryana). Although other types of ritual language, such as the Buddhist dharani or vidya, are distinguished from mantras, all of these types are sometimes used interchangeably or without any formal basis for distinction.
The practice of mantra coincides with an ancient Indian belief in the sacred power of word or sound, developed in such later texts as Bhartrhari's Vakyapadiya (5th c AD). Mantras are used both for worship and meditation, and for practical or magical purposes including causing rain or attracting members of the opposite sex. In Tantric ritual, the mantra must be received from one's teacher (guru) in order to be effective, and is used either by itself in recitation (japa) or in combination with other practices and implements, such as gestures (mudra), diagrams (yantra, mandala), and fire sacrifices (homa).
Mantras must be repeated in precise form to be ritually effective. They have conventional endings and beginnings, including especially the syllable 'om' and often include vocables, such as the 'seed' syllables (bijas) prevalent in Tantric mantras, which lack the semantic value of ordinary words. For these and other reasons, some observers, including the Vedic ritualist Kautsa and the Buddhist philosopher Vasubandhu (4th century AD), concluded that mantras are meaningless. Whatever the case may be, it is clear that mantras exhibit numerous stylistic devices found in both poetry and the ritual languages, spells, and chants of other cultures, and that bijas resemble other repetitive and 'nonsensical' magic words found around the world. [Robert A Yelle]