Ayatollah is an honorific title for high-ranking Twelver Shia clergy. it 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". From Wikipedia
