Ayatollah (Arabic: آية الله; Persian: آيت‌الله); is a high ranking title given to major Shi'a clerics. The word means "Sign of God" and those who carry the title are experts in Islamic studies such as jurisprudence, ethics, and philosophy and usually teach in seminaries (Hawzas) of Islamic sciences. The next lower clerical rank is Hojjat-ol-Eslám ("Authority on Islam").

The rank is granted by consensus, rather than ceremonially: an esteemed religious scholar who has earned the respect and admiration of his teachers for his knowledge and behavior after completing his howze studies. By then he would be able to issue his own edicts from the sources of religious laws: Qur'an, Sunnah, Ijmāˤ and Aql "intellect" (the equivalent of the Sunnī principle of Qiyas). Most of the time this is attested by an issued certificate from his teachers. This Ayætollah then can teach in religious howzeha according to his speciality and can act as a reference for their religious questions and as a judge.

Only a few of the most important Ayatollah are accorded the rank of Grand Ayatollah (Áyatolláh e Ozmá, "Great Sign of God") or Marjà e Taqlid ("Reference for Emulation"). This usually happens when the followers of one of the Ayatollahs refer to him in many situations and ask him to publish his Juristic book in which he answers the vast majority of daily Muslim affairs. The book is called Resále e Elmiye, which is usually a reinvention of the book Al-Urwatu l-Wuthqah according to their knowledge of the most authentic Islamic sources and their application to current life.

There is usually one Grand Ayatollah in Iraq that heads the Hawzas (currently Ali Sistani) and a few that coordinate with him like Mohammad Said Al-Hakim, Mohammad Ishaq Al-Fayyad, and Mohammad Taqi al-Modarresi. There are more in Iran and wherever the Shi'a exist. There are more than 20 living world-wide; the most famous of them are Naser Makarem Shirazi, Ali Khamenei, Ali Sistani, Kazem al-Haeri, Sadeq Rohani, Fazel Lankarani, Montazeri, Mohammad Hussein Fadlullah, and Sadiq Hussaini Shirazi.

There are a few females which are equal in ranking to the Ayatollahs, and are known as Lady Mujtahideh. An example of which is Lady Mujtahideh Zohreh Sefati (official website). The first Lady Mujtahideh was Nosrat Amin Esfhani, born in 1886 to a religious family, she received her Ijtihad degree from Grand Ayatollahs of Qom at the age of 40.

When Westerners say "the Ayatollah", they usually mean Ayatollah Ruhollah Khomeini, who brought the word into the international limelight during the 1979 Iranian Revolution; a possible secondary meaning would be the term's use to indicate the serving Supreme Leader of Iran.

"The Ayatollah" is also the name of a celebration performed by Cardiff City fans during a game. The celebration itself consists of using both arms to pat the head at the same time, and is derived from media coverage of the mourning displayed when Ayatollah Khomeini died, and originally stemmed from the grief of watching Cardiff City in the lower leagues, it has now transcended this and is regularly seen on the terraces and as a celebration by sportsmen in other sports who wish to show their support of Cardiff City. Wales rugby captain Gareth Thomas performs the celebration when he scores a try, Glamorgan and England cricket player Simon Jones also used the celebration when he took the wicket of Simon Katich in the first innings of the fourth Test of the 2005 Ashes Series.