Mananat-trayate iti Mantrah. Mantra means anything which protects you when you remember it constantly, think about it constantly and recollect it and recite it.
Hence the supreme god also has the name Mantra.
Listen to talks about Hinduism - The sanatana dharma , about pages from Puranas, upanishads and mantra shastras and everything that protects us when we think about it, discuss about it and and learn about it