Today the terms “Sunni” and “Ahl al-Sunna” are applied to the majority of Muslims living in the world, chiefly in opposition to the Shiʿis and some other Muslim groups. As such, most introductions to and histories of Islam implicitly equate “Muslim” with “Sunni,” unless otherwise specified. The contemporary usage of the term is sometimes a-historically projected into the past, such that starting from the fist Islamic century onward, everyone who was not a Shiʿi, a Khariji, a Murji’i, a Muʿtazili, etc., is often called by scholars “Sunni.” The historical record shows, however, that such a usage of the term is uncritical and inaccurate. Not only did “Sunni,” “Ahl al-Sunna,” or “Ahl al-Sunna wa l-Jama’a” not mean the same thing in the early centuries of Islam as they did later, but their meaning was at times so fluid that – as late as in the 3rd/9th century – authors of diametrically opposed orientations each claimed these terms to represent his community. This included a hadith scholar, a Muʿtazili, and an Ismaili, no less. It is only a century or so later that the terms “Sunni” and “Ahl al-Sunna” assumed the meaning that is known to this day. Namely, they came to denote the majority of Muslims who adhered to one of the four legal schools of law, espoused certain theological tenets, and who now are often presumed to represent what may be termed as “Muslim orthodoxy” (the problematic nature of the latter term is well-known). This was not a spontaneous development, but the result of inter-sectarian polemics and a struggle for power and prestige in the Abbasid caliphate. Through the analysis of historical, heresiological and other texts, my paper will examine the history of the term Sunni (or “Ahl al-Sunna”) and the religio-political conditions that led to the crystallization of its usage.