Twelver or Imami Shīa Islam (Athnā‘ashariyyah or Ithnā‘ashariyyah, Arabic: اثنا عشرية) is the largest branch of Shī‘ī (Shi'a) Islam. Adherents of Twelver Shī‘ism are commonly referred to as Twelvers, which is derived from their belief in twelve divinely ordained leaders, known as the Twelve Imāms and their belief that the Mahdi will be none other than the returned Twelfth Imam that disappeared and is believed by Twelvers to be in occultation. Approximately 85% of Shī‘a are Twelvers, and the term Shi'a Muslim as commonly used in English usually refers to Twelver Shī‘a Muslims only.
Twelvers share many tenets of Shī‘ism with related sects, such as the belief in Imāms, but the Ismā‘īlī and Zaydī Shī‘ī sects each believe in a different number of Imāms and for the most part, a different path of succession regarding the Imāmate. They also differ in the role and overall definition of an Imām.
The Twelver faith is a majority in countries like Iran, Iraq, Azerbaijan, Bahrain, and Lebanon. Alevis in Turkey and Albania and Alawis of Syria also regard themselves as Twelvers, but hold significantly different beliefs from mainstream Twelver Shiites. The Twelver faith also forms a large minority in India,Pakistan, Afghanistan, Nigeria, Tanzania, Kuwait, Qatar, UAE and Saudi Arabia. Smaller minorities of Twelver also exist in Oman, Yemen, Egypt, Sudan, Kenya, Ghana, Senegal, Indonesia, Malaysia and in many other countries of the world, including Europe and the Americas.