Overview
Ayatollah is an honorific title for high-ranking Twelver Shia clergy in Iran that came into widespread usage in the 20th century. Originally used as a title bestowed by popular/clerical acclaim for a small number of the most distinguished marja' at-taqlid mujtahid, it suffered from "inflation" following the 1979 Iranian Revolution when it came to be used for "any established mujtahid". By 2015 it was further expanded to include any student who had passed their Mujtahid final exam, leading to "thousands" of Ayatollahs.
The title is not used by the Sunni community of Iran, nor by Shias in Lebanon, Pakistan, or India. In Iraq, the title is not unknown, but is only used for clerics of Iranian origin.
In the Western world – especially in decade after the Iranian Revolution – it was associated with Ruhollah Khomeini, who was so well known as to often be referred to as "The Ayatollah".
Etymology
The title is originally derived from Arabic word pre-modified with the definite article and post-modified with the word Allah, making. The combination has been translated to English as 'Sign of God', 'Divine Sign' or 'Reflection of God'. It is a frequently-used term in Quran, but its usage in this context is presumably a particular reference to the verse "We shall show them Our signs on the horizons and in their own selves", while it has been also used to refer to The Twelve Imams by Shias.
From Wikipedia (CC BY-SA 4.0).