Sacred sound in the
most fundamental sense.
A mantra is a collection of phonemes that may or may not
have grammatical meaning as real words, since its significance stems from the
sounds themselves rather than the meaning of the utterances.
Those who have been granted the qualification (adhikara) to
employ mantras are said to gain strength and other spiritual abilities.
The qualification comes from being handed the mantra by
one's instructor, which is said to impart not just the mantra's tones but also
its potency.
This live transmission is seen to be an important aspect of
"possessing" the mantra; as a result, mantras learnt in other
settings are thought to be ineffectual.
The Vedas, the earliest Hindu religious books, first mention
mantra as a holy sound.
The Gayatri mantra is a passage from the Rig Veda and is one
of the most popular mantras (3.62.10).
Tantra is a hidden ritual-based religious discipline that
emphasizes the use of mantras.
See Arthur Avalon's (Sir John Woodroffe's) Shakti and
Shakta, 1978; Swami Agehananda Bharati's The Tantric Tradition, 1977; and
Douglas Renfrew Brooks' The Secret of the Three Cities, 1990 for further
information.
You may also want to read more about Hinduism here.
Be sure to check out my writings on religion here.