What is "zaidiyyah"

Zaidiyyah or Zaidism ( az-zaydiyya, adjective form Zaidi or Zaydi) is an early sect which emerged in the eighth century out of Shi'a Islam, named after Zayd ibn ʻAlī, the grandson of Husayn ibn ʻAlī. Followers of the Zaydi Islamic jurisprudence are called Zaydi Shi'a and make up about 35-40% of Muslims in Yemen.