mantra is a means of expressing thoughts of devotion or prayer.
I've been revisiting my notes on mantra for my Heart Sutra book and made an interesting discovery.The etymology of mantra has always puzzled me. The ancient grammarians derived it from a root √mantr, "mantra" in this view is a simple verbal noun with the suffix -a (Cf. Apte's dictionary definition which gives the Pāṇinian code mantr-aC).
However, modern grammarians are almost unanimous in rejecting this. They consensus is that the root is √man which is usually said to mean "think". This gave rise to the idea that mantra combined the root with an instrumental suffix -tra to mean something like "a tool for thinking". The trouble with this is that no one ever used a mantra as a tool for thinking. If anything the opposite is true, mantras are tools for avoiding thoughts. The first mantras were verses from the Ṛgveda used as liturgy in rituals. Like the the Gāyatrī mantra, from the Ṛgveda (ṚV 3.57.10)
tat savitur vareṇyaṃ bhargo devasya dhīmahi dhiyo yo naḥ pracodayāt
We meditate on the lovely Glory of the god Savitṛ That he may stimulate our minds. (Roebuck's translation)
Note the word for meditate here is dhīmahi which is related to the word dhyāna (dhī + āna > dhyāna)
Trying to find a better explanation, I returned to a book that I have long valued for its insights into the Vedic religious imagination, William K Mahony's The Artful Universe (he's an academic and high-level haṭha yoga teacher). Mahony draws attention to an aspect of Sanskrit words from √man that I had previously overlooked, but which is confirmed by standard dictionaries. √man does mean "think", but more importantly the primary meaning is "the verbal expression of thought". The job of the ṛṣi or seer, was not simply to find meaning and significance in the mundane, but to give it verbal expression in order to make it accessible to everyone.
The Ṛgveda was an anthology of such verses, collated around 1500-1200 BCE, which are intended give meaning to life for Brahmins. And to some extent they still do, though more in theory than in practice.
So words like mati do mean "thinking, thought, mind" etc, but more importantly mati means "prayer, devotion". A manman is a verbal expression of a mental image. A maniṣā is a verbal insight. So a mantra is not “tool for thinking”, but rather mantra is a means of expressing thoughts of devotion or prayer.
This is far more satisfying as a definition since this is how people actual use mantras, and it is still consistent with etymology and dictionaries. Note that this also explains also the most important beliefs of Brahmanism are referred to as śruti "something heard" even though those who transmit such beliefs are called ṛṣi "seer". Like all priests, the ṛṣi plays a mediating role between the seen and the unseen worlds that people believed in.
One of the main ways the ṛṣi had their visions was to combine mediation with sleep deprivation brought on by the use of stimulants (probably ephedra). This is quite a dangerous practice, which can induce seizures and/or psychosis, and can be fatal if prolonged by more than a few days. So don't try it lightly or without supervision.