Answer
The honorific "Mírzá" is used in Persian as a title of respect for male descendants of the Prophet Muhammad or for learned men. ʻAbdu'l-Bahá was the eldest son of Baháʼu'lláh, the founder of the Baháʼí Faith, and was considered a direct descendant of Muhammad through his daughter Fatimah. As a result, he was commonly referred to as "Mírzá ʻAbbás" in Persian, acknowledging his noble lineage and scholarly status.