"אדמו"ר הזקן" (Admor HaZaken) refers to Rabbi Shneur Zalman of Liadi, the founder of the Chabad-Lubavitch movement and the author of the Tanya and the Shulchan Aruch HaRav. He is also known as the Baal HaTanya and Baal HaShulchan Aruch. The title "Admor HaZaken" translates to "The Elder Rebbe," distinguishing him from later Rebbes in the Chabad dynasty. His teachings laid the foundation for Chabad Chassidus, emphasizing the intellectual approach to divine service, known as "Chabad" (an acronym for Chochmah, Binah, and Daas - Wisdom, Understanding, and Knowledge).