Shi'a: A member of the smaller of the two primary branches of orthodox Islam, the other being Sunnism. The three primary sects of Shi'ism are the Twelvers, Ismailis, and Zaidis.
Islam split into Sunnis and Shi'as during the first Islamic civil war, 656-661. The Shi'a followed Muhammad's cousin and son-in-law Ali, while the Sunnis followed the caliphs instead.
The word Shi'a comes from the Arabic sh'a, meaning "a following or sect," from the root say'a meaning "to spread."
"The Shi'a are the most common sect in Iran."