Sense: ‘A sacred sound phoneme, single syllable or group of energetically charged words, which when chanted, effect changes in consciousness, bodily energy and quality of the soul.’

Root: Man—the thinking principle; tra—instrument (with the implication of protection); thus a man-tra is that instrument which protects one from one’s own thoughts. Various pandits or scholars, consider the word comes from other roots and take it to mean ‘that which has been thought or known’ or ‘that which is received by initiation (d⁄k‚å) and has the power to liberate.’

One who chants a mantra is called a mantrim.

Brahmanic traditionalism designates only those verses of the Vedas which were heard by rishis in deep meditation as true mantra. Some scholars narrow it down to only the Samhita section of the Veda. However, sacred sounds called bijas or ‘seed sounds’ and other formula intuited by Shaivite saints and Tantrics are also accepted as true mantra by other schools of thought.
From: Words on the Way by Muz Murray