Atman
आत्मन् (ātman)
The self behind the selves - the unchanging witness the Upanishads point to beneath body, mind, and role.
Atman began as a reflexive pronoun - oneself - and became the Upanishads' central quarry: what, finally, is the self? Not the body, which changes; not the mind, whose contents parade past; not the roles, which are costume. What remains is the witness of all of these, itself unwitnessed.
The famous equation atman is brahman - the innermost self is not other than the ground of everything - is Vedanta's most audacious sentence, and its schools spent centuries specifying what it does and does not mean.
As practice rather than doctrine, atman functions as a question: the meditative habit of asking, of every experience, is this what I am, or something I am watching? The question does not require an answer to do its work. It loosens identifications simply by being asked sincerely.