Adi Shankara was an influential Indian philosopher and theologian who lived in the 8th century CE, best known for consolidating the doctrine of Advaita Vedanta, which emphasizes non-dualism and the unity of the individual soul (Atman) with the ultimate reality (Brahman). His teachings and writings played a crucial role in the revival of Hindu philosophy and spirituality at a time when Buddhism was declining in India, leading to a renewed focus on Vedantic thought and practice.