The atman is variously translated into English as the eternal self, spirit, essence, soul, or breath. It is the true self as opposed to the ego; that aspect of the self which transmigrates after death or becomes part of Brahman (the force underlying all things). The final stage of moksha (liberation) is the understanding that one's atman is, in fact, Brahman.
The concept of the atman is central to all six major schools of Hinduism, and it is one of the major differences between Hinduism and Buddhism. Buddhist belief does not include the concept of the individual soul.