Sunni and Shia names derive from Arabic for custom and party

Evidence

In time, those who accepted the legitimacy of the early caliphs came to be known as Sunni, from “sunna,” an Arabic word meaning usage or custom, and applied particularly to the body of precedent constituted by the actions and utterances of the Prophet and his immediate successors. These, handed down by tradition, were regarded as legally and religiously binding in Sunni Islam. Those opponents who followed Ali and his descendants came to be known by another Arabic word, Shi a, meaning party or following—at first as the Shi a of Ali, and then simply as the Shi a. The individual adherent to this cause was called a Shi i, or in common English usage, Shi ite. — Bernard Lewis, From Babel to Dragomans, 1425

Categorization

• DEFINE: Clarifies etymology of Sunni (custom/precedent) and Shia (party/following) and the proper forms Shia-of-Ali, Shii, Shiite.
• corroborated: 2 independent passages support this claim — Bernard Lewis, From Babel to Dragomans: Interpreting…, 1425