What does "Advaita" actually mean?

Advaita is Sanskrit for "not-two" — non-dual. Advaita Vedanta is the school of Indian philosophy built around one central claim: that ultimate reality is a single, undivided consciousness, usually called Brahman, and that the felt sense of being a separate, limited individual — this body, this personality, cut off from everything else — is not the final truth about you, even though it's how things ordinarily appear. It was systematized primarily by Adi Shankaracharya, a philosopher and teacher who lived around the 8th century CE, through commentaries on three foundational texts collectively called the Prasthanatrayi: the Upanishads (the older philosophical portions of the Vedas), the Bhagavad Gita, and the Brahma Sutras. Shankara didn't invent the ideas from nothing — he argued they were already there in these older texts — but he's the figure most responsible for Advaita becoming a coherent, defensible philosophical system. The common misreading is that Advaita says the world is an illusion in the sense of being nothing at all, so nothing you do matters. That's not the actual claim. The world isn't said to be non-existent — you experience it, actions in it have real consequences, ethics apply fully within it. What Advaita says is that the world's apparent separateness — you as one thing, that as another thing, over there, distinct from everything else — is not the deepest layer of reality. Underneath the multiplicity, the claim goes, there is one undivided reality appearing as many, the way one ocean appears as many waves without ever actually stopping being one ocean. That's the whole starting claim. Everything else in Advaita — the ideas of Atman, Maya, self-inquiry — is really just working out what follows if that one claim is true.