Hayyi Rabbi, "the Great Life", is the supreme, ultimately transcendent divine principle of Mandaean theology, the source from which the Lightworld and the whole subsequent chain of divine beings, the uthras, emanate outward, situated at the very summit of Mandaean cosmology above every other named divine figure the tradition recognises.
Jorunn Jacobsen Buckley's study of Mandaean texts and the modern Mandaean community describes Hayyi Rabbi as functioning less like a personal god a worshipper addresses directly with individual petitions and more like the ultimate source and standard of life, light and truth against which the whole of Mandaean cosmology, ritual and ethical teaching is oriented, invoked constantly through the term "Life" itself in Mandaean prayer and ritual formula rather than approached as a distant, separately named deity. Mandaeans, who call themselves Mandayya, "knowers", after manda, gnosis or knowledge, understand correct ritual practice, particularly frequent baptism in flowing water, masbuta, as the primary means of maintaining connection to Hayyi Rabbi and the Lightworld the Great Life represents.