Om

the sound that is not made by two things striking together

The symbol you see here, known as "Om", is often found in tattoos inspired by Hindu philosophy. Few people, however, really know what it means, they just assume it must be cool.

To simplify: Everything is made up of vibrating energy. It's similar to sound, with a fundamental difference -- the sounds that we hear with our ears are made by two objects striking each other on some level. Sound that is not made by two objects striking each other is the sound of this energy -- it is said that this "sound" can be best analogized as the sound OM (pronounced: silence, then AH-OOO-MMMM, then silence).