The syllable Aum is first described as all-encompassing mystical entity in the Upanishads. Today, in all Hindu art and all over India and Nepal, 'Aum' can be seen virtually everywhere, a standard sign for Hinduism and its philosophy and mythology.
Aum is not a word but rather an intonation, which, like music, transcends the barriers of age, race, culture and even species. It is made up of three Sanskrit letters, aa, au and ma which, when combined together, make the sound Aum or Om. It is believed to be the basic sound of the world and to contain all other sounds. If repeated with the correct intonation, it can resonate throughout the body so that the sound penetrates to the centre of one's being, the atman or soul.
In modern time, Aum tattoos have become very popular with the crowd, regardless of thier age, sex or even religion. It has become the most common design to my knowledge and seriously I have lost count on how many of my clients carry that sacred symbol.