The
realization of yourself as an
individual. Being able to seperate yourself from the
environment around you. Being able to
think that you
exist.
Rather hard to define, actually, when you think about it.
The ability to look in a mirror and realize that's a reflection of you, and not another being, is a big sign of being self aware. There is a clear line in the animal kingdom between animals that understand mirrors and those that don't.