The notion of bayān lit. ‘clarity, distinctness’ in classical Arabic-Islamic writings is, as Wolfhart Heinrichs noted, “not clear and distinct at all and in dire need of a monograph.” Although his words still largely hold true, there have been advances in the study of this term, especially with regards to the legal work of al-Shāfiʿī (Lowry, Montgomery, Vishanoff) and the literary-linguistic work of al-Jāḥiẓ (Montgomery, Skarżyńska-Bocheńska, Suleiman, Behzadi). The phrase ʿilm al-bayān ‘the science/knowledge of bayān’, however, has yet to be studied. In this paper I discuss the nature of ʿilm al-bayān as a discipline heading and explore its relation to the indigenous understanding(s) of the term bayān. I show that the phrase ʿilm al-bayān denotes two scholarly endeavors rather than one, and that each one stems from a different conception of the term bayān. The phrase ʿilm al-bayān seems to have appeared ex nihilo in the introduction to the work Dalāʾil al-iʿjāz by the literary theorist ʿAbd al-Qāhir al-Jurjānī (5th/11th c.), and scholars have correctly taken it as a non-technical designation (Abu Deeb, Larkin). I examine how the phrase was later used in the work of al-Zamakhsharī and Fakhr al-Dīn al-Rāzī (6th/12th c.), and how it was subsequently appropriated in the 7th/13th century by al-Sakkākī on one hand and Ḍiyāʾ al-Dīn Ibn al-Athīr on the other. For al-Sakkākī, bayān was associated with the study of imagery – a reading that is not borne out in the earlier works of al-Zamakhsharī and al-Rāzī. Unraveling this association is complicated by the fact that none of the numerous commentators in the “Sakkākian” tradition explain the specific technical meaning of the term bayān in the phrase ʿilm al-bayān; this information must be gleaned from earlier philological works. In the case of Ibn al-Athīr, textual evidence suggests a separate understanding of the term bayān in the larger ʿilm al-bayān, one pertaining to scribal writing style and conventions. Ibn al-Athīr’s version of ʿilm al-bayān triggered a flourishing of works on ʿilm al-bayān in the Arabic East which lasted well beyond the time of the Syrian al-Qazwīnī (8th/14th c.).