The Ātman (IAST: Ātman, sanskrit: आत्मन्) is a philosophical term used within Hinduism and Vedanta to identify the soul whether in global sense (world's soul) or in individual sense (of a person own soul). It is one's true self (hence generally translated into English as 'Self') beyond identification with the phenomenal reality of worldly existence.
Philosophical schools such as Advaita (monism) see the soul within each living entity as being fully identical with Brahman - the all-pervading soul of the universe, whereas other schools such as Dvaita (dualism) differentiate between the individual atma in living beings, and the Supreme atma (Paramatma) as being at least partially separate beings. Thus atman refers to the individual soul or the observer being.
Within Advaita Vedanta philosophy the Atman is the universal life-principle, the animator of all organisms, and the world-soul. This view is of a sort of panentheism (not pantheism) and thus is sometimes not equated with the single creator God of monotheism. Identification of individual living beings/souls, or jiva-atmas, with the 'One Atman' is the monistic Advaita Vedanta position, which is critiqued by dualistic/theistic Dvaita Vedanta. Dvaita Vedanta calls the all-pervading aspect of Brahman Paramatman quantitatively different from individual Atman and claims reality for both a God functioning as the ultimate metaphorical "soul" of the universe, and for actual individual "souls" as such. The Dvaita, dualist schools, therefore, in contrast to Advaita, advocate an exclusive monotheistic position wherein Brahman is made synonymous with Vishnu. Aspects of both philosophies are found within the schools of Vishishtadvaita Vedanta and Achintya Bheda Abheda.
In some instances both Advaita and Dvaita schools may accommodate the others's belief as a lower form of worship or practice towards the same ultimate goal. 
In the view of the Yoga school, the highest attainment does not reveal the experienced diversity of the world to be illusion. The everyday world is real. Furthermore, the highest attainment is the event of one of many individual selves discovering itself; there is no single universal self shared by all persons.
The pre-Buddhist Upanishads link the Self to the feeling "I am." Others like the post-Buddhist Maitri Upanishad hold that only the defiled individual self, rather than the universal self, thinks "this is I" or "this is mine".
Adherents to Jainism also use the phrase the atman to refer to 'the self'. Often atman is mistaken as being interchangeable with the word jiva with the difference being somewhat subtle. Whereas atman refers to the self, jiva refers to the living being, the exact comprehension of which varies throughout the philosophical schools.