Aum, or Om, is the most used word in Hindi religion, as it represents the union of the three gods – Brahma, Vishnu and Shiva. It also represents creation and originates from the holy sanskrit language. It is used vocally in meditation, at temples and shrines (as in “Aum namo Shiva”), and also as a greeting to holy men (if you happen to come across a sadhu  don’t say “Namaste”, instead you should say “Aum namo narayan” it’s a bit of a mouthful but is more respectful to them.