Zaidiyyah or Zaidism (Arabic: الزيدية az-zaydiyya, adjective form Zaidi or Zaydi) is a Muslim sect which emerged in the eighth century out of Shi'a Islam. Zaidis are named after Zayd ibn ʻAlī, the grandson of Husayn ibn ʻAlī who they recognize as the fifth Imam. Followers of the Zaydi Islamic jurisprudence are called Zaydi Shi'a and make up about 35-40% of Muslims in Yemen. Zaidis dismiss religious dissimulation (taqiyya).
The Zaydi madhab emerged in reverence of Zayd’s failed uprising against the Ummayad Caliph, Hisham ibn Abd al-Malik (ruling 724-743 AD), which set a precedent for revolution against corrupt rulers. It might be said that Zaydis find it difficult to remain passive in an unjust world, or in the words of a modern influential Zaydi leader, Hussein Badreddin al-Houthi, to ‘sit in their houses’.
Zaydis were the oldest branch of the Shia and are currently the second largest group after Twelvers . Zaidis do not believe in the infallibility of Imāms, but promote their leadership and divine inspiration. Zaydis believe that Zayd ibn Ali in his last hour was betrayed by the people in Kufa. Zaydis as of 2014 constitute roughly 0.5% of the world's Muslim population.
In matters of Islamic jurisprudence, the Zaydis follow Zayd ibn ’Ali's teachings which are documented in his book Majmu’ al-Fiqh (Arabic: مجموع الفِقه). Zaydi fiqh is similar to the Hanafi school of Sunni Islamic jurisprudence.Abu Hanifa, a Sunni madhab shaykh, was favorable and even donated towards the Zaydi cause.