'Mananat-trayate iti Mantrah' - By the Manana (constant thinking or recollection) of which one is protected or is released from the round of births and deaths, is Mantra. That is called Mantra by the meditation (Manana) on which the Jiva or the individual soul attains freedom from sin, enjoyment in heaven and final liberation, and by the aid of which it attains in full the fourfold fruit (Chaturvarga), i.e., Dharma, Artha, Kama and Moksha. A Mantra is so called because it is achieved by the mental process. The root 'Man' in the word Mantra comes from the first syllable of that word, meaning 'to think', and 'Tra' from 'Trai' meaning 'to protect' or 'free' from the bondage of Samsara or the phenomenal world. By the combination of 'Man' and 'Tra' comes Mantra.
A Mantra is divinity encased within a sound-structure. It is divine power or Daivi Sakti manifesting in a sound-body. The Mantra is itself Devata.
The sacred Mantra or the Divine Name is a vital symbol of the Supreme Divinity directly revealed in the innermost depths of divine communion to the sages of Self-realization in the hoary Vedic and Upanishadic times. These symbols are in the nature of unfailing keys to gain access into the transcendental realms of absolute experience.
Mantra Yoga is an exact science. A Mantra, in the Hindu religion, has the following six parts. It has got a Rishi (a man of Self-realization) to whom it was revealed for the first time and who gave this Mantra to the world. He is the Drashta or Seer for this Mantra. Sage Viswamitra is the Rishi for Gayatri. Secondly, the Mantra has a metre (Chhandas), which governs the inflection of the voice. Thirdly, the Mantra has a particular Devata or supernatural being, higher or lower, as its informing power. This Devata is the presiding deity of the Mantra. Fourthly, the Mantra has got a Bija or seed. The seed is a significant word, or series of words, which gives a special power to the Mantra. The Bija is the essence of the Mantra. Fifthly, every Mantra has got a Sakti. The Sakti is the energy of the form of the Mantra, i.e., of the vibration-forms set up by its sound. These carry the man to the Devata that is worshipped. Lastly, the Mantra has a Kilaka - pillar or pin. This plugs the Mantra-Chaitanya that is hidden in the Mantra. As soon as the plug is removed by constant and prolonged repetition of the Name, the Chaitanya that is hidden is revealed. The devotee gets Darshana of the Ishta Devata.
SOUND AND IMAGE
Sounds are vibrations. They give rise to definite forms. Each sound produces a form in the indivisible world, and combinations of sound create complicated shapes. Repetition of a Mantra has a mysterious power of bringing about the manifestation of the Divinity, just as the splitting of an atom manifests the tremendous forces latent in it. When a particular Mantra appropriated to a particular god is properly recited, the vibrations so set up create in the higher planes a special form which that god ensouls for the time being. The repetition of the Panchakshara Mantra - Om Namo Sivaya - produces the form of Lord Siva. The repetition of Om Namo Narayanaya, the Ashtakshara Mantra of Vishnu, produces the form of Vishnu.
GLORY OF DIVINE NAME
The Name of God, chanted correctly or incorrectly, knowingly or unknowingly, carefully, is sure to give the desired result. Just as burning quality is natural to and inherent in fire, so also, the power of destroying sins with their very root and branch, and bringing the aspirant into blissful union with the Lord through Bhava-Samadhi, is natural to and inherent in the Name of God.
The glory of the Name of God cannot be established through reasoning and intellect. It can be experienced or realized only through devotion, faith and constant repetition of the Name.
There is a Sakti or power in every word. If you utter the word 'excreta' or 'urine' when your friend is taking his meals, he may at once vomit his food. If you think of 'Garam Pakoda', 'hot Pakoda' (fried delicacies), your tongue will get salivation. When anyone suddenly shouts 'Scorpion! Scorpion!', 'Snake! Snake!', you at once apprehend the scorpion or the snake and jump in fright. When anyone calls you a 'donkey' or an 'ass', you are annoyed and you show anger. If anyone says, "You are a nice person," you smile. When such is the power of the names of the ordinary things of this world, what tremendous power should there be in the Name of God! God is the completion or the fullness of existence. Hence, the Name which denotes Him, too, is full and perfect. Therefore, the power of the Name of God is incalculable, for it is the height or the zenith of power. The Name of God can achieve anything. There is nothing impossible for it. It is the means to the realization of God Himself. Even as the name of a thing in this world generates the consciousness of that thing in the mind, the Name of God generates God-consciousness in the purified mind and becomes the direct cause of the realization of the Highest Perfection or God.
This is reproduced from the book, "Japa Yoga" by Swami Sivananda