Sayyid is an honorific title of Hasanid and Husaynid lineage, recognized as descendants of the Islamic prophet Muhammad through his daughter Fatima and Ali's sons Hasan and Husayn. The title may also refer to the descendants of the family of the Bani Hashim through the Prophet's great-grandfather Hashim, and others including Hamza, Abbas, Abu Talib, and Asad ibn Hashim.
See also Sharif, which has a good description of three types of identification for that term and co-relates to this term. In its narrowest sense, a sayyid is a descendant of Husayn ibn Ali, but the term is also more generally used for descendants of The Prophet.
