Isma'ilism is a branch or sect of Shia Islam. The Isma'ili get their name from their acceptance of Imam Isma'il ibn Jafar as the appointed spiritual successor to Ja'far al-Sadiq, wherein they differ from the Twelver Shia, who accept Musa al-Kazim, the younger brother of Isma'il, as the true Imām.
