"Twelver" is a term which is used to describe the majority of the adherents of the Shi'a branch of the Islamic faith. The descriptive word "Twelver" is derived from the fact that most Shi'ite Muslims hold to the belief that since Islam was founded by Muhammad during the early 7th Century, there have been twelve divinely ordained leaders of the faith, who are referred to as the "Twelve Imāms".
A principle belief amongst the Twelvers is that the Twelfth Imām -- Muhammad al-Mahdi -- disappeared, and is currently in occultation, meaning that he is alive, but hidden from public view. According to Shi'ite Muslim belief, sometime in the near future, the Twelfth Imām will return as the long awaited Muslim messiah; that is, the Mahdi. The Twelver branch of Shi'ite Islam is prominent in Iran, Iraq, Azerbaijan, Turkey, Lebanon and Bahrain. There are also large minorities in India, Pakistan, Afghanistan, Kuwait and Saudi Arabia.
Please also refer to the term "mahdi" for additional information.