'Mantra (Skt., Tib. ngag): Natural vibration of a buddha aspect; activates the Buddha aspect's power-field. Many Diamond Way meditations have a phase where mantras are repeated.'
-- Quoted from 'Fearless Death' by Lama Ole Nydahl.
Blog author's comments.
Mantra is also protecting sound (sound is conveyed by air's vibration).
i think that there can be visual Mantras as well (conveyed by light - an electromagnetic oscillation of a certain frequencies).
Perhaps there's more.
On a certain experiencing level, all sounds are mantras, all thoughts are wisdom.