Ātman is the Sanskrit word for the self. The Upaniṣads use it for what is deepest in a person: not the body, the senses or the mind, but the conscious witness of them all. Their central teaching is that this self is not different from Brahman, the ground of the universe. The schools of Vedānta interpret the relation differently, as identity, as a part within a whole, or as dependence.
Ātman
The self: in the Upaniṣads, the innermost reality of a person, which is said to be one with Brahman.
Also: Atman, the Self